Improve the performance of the GET /:sources/:id/{access_requests,members} API endpoints

The performance was greatly improved by removing two N+1 queries issues
for each endpoint.

For comparison:

1. `GET /projects/:id/members`

With two N+1 queries issues (one was already fxed in the following
snippet):

```
  ProjectMember Load (0.2ms)  SELECT "members".* FROM "members" WHERE
"members"."source_type" = $1 AND "members"."type" IN ('ProjectMember')
AND "members"."source_id" = $2 AND "members"."source_type" = $3 AND
"members"."type" IN ('ProjectMember') AND "members"."requested_at" IS
NULL  ORDER BY "members"."id" DESC  [["source_type", "Project"],
["source_id", 1], ["source_type", "Project"]]
  User Load (0.5ms)  SELECT "users".* FROM "users" WHERE "users"."id" IN
(5, 22, 16, 13)  ORDER BY "users"."id" DESC
  ActiveRecord::SchemaMigration Load (0.2ms)  SELECT
"schema_migrations".* FROM "schema_migrations"
  ProjectMember Load (0.3ms)  SELECT  "members".* FROM "members" WHERE
"members"."source_type" = $1 AND "members"."type" IN ('ProjectMember')
AND "members"."source_id" = $2 AND "members"."source_type" = $3 AND
"members"."type" IN ('ProjectMember') AND "members"."requested_at" IS
NULL AND "members"."user_id" = $4  ORDER BY "members"."id" DESC LIMIT 1
[["source_type", "Project"], ["source_id", 1], ["source_type",
"Project"], ["user_id", 5]]
  ProjectMember Load (0.3ms)  SELECT  "members".* FROM "members" WHERE
"members"."source_type" = $1 AND "members"."type" IN ('ProjectMember')
AND "members"."source_id" = $2 AND "members"."source_type" = $3 AND
"members"."type" IN ('ProjectMember') AND "members"."requested_at" IS
NULL AND "members"."user_id" = $4  ORDER BY "members"."id" DESC LIMIT 1
[["source_type", "Project"], ["source_id", 1], ["source_type",
"Project"], ["user_id", 22]]
  ProjectMember Load (0.3ms)  SELECT  "members".* FROM "members" WHERE
"members"."source_type" = $1 AND "members"."type" IN ('ProjectMember')
AND "members"."source_id" = $2 AND "members"."source_type" = $3 AND
"members"."type" IN ('ProjectMember') AND "members"."requested_at" IS
NULL AND "members"."user_id" = $4  ORDER BY "members"."id" DESC LIMIT 1
[["source_type", "Project"], ["source_id", 1], ["source_type",
"Project"], ["user_id", 16]]
  ProjectMember Load (0.3ms)  SELECT  "members".* FROM "members" WHERE
"members"."source_type" = $1 AND "members"."type" IN ('ProjectMember')
AND "members"."source_id" = $2 AND "members"."source_type" = $3 AND
"members"."type" IN ('ProjectMember') AND "members"."requested_at" IS
NULL AND "members"."user_id" = $4  ORDER BY "members"."id" DESC LIMIT 1
[["source_type", "Project"], ["source_id", 1], ["source_type",
"Project"], ["user_id", 13]]
```

Without the N+1 queries issues:

```
  ProjectMember Load (0.3ms)  SELECT  "members".* FROM "members" WHERE
"members"."source_type" = $1 AND "members"."type" IN ('ProjectMember')
AND "members"."source_id" = $2 AND "members"."source_type" = $3 AND
"members"."type" IN ('ProjectMember') AND "members"."requested_at" IS
NULL  ORDER BY "members"."id" DESC LIMIT 20 OFFSET 0  [["source_type",
"Project"], ["source_id", 1], ["source_type", "Project"]]
  User Load (0.5ms)  SELECT "users".* FROM "users" WHERE "users"."id" IN
(5, 22, 16, 13)  ORDER BY "users"."id" DESC
```

2. `GET /projects/:id/access_requests`

With two N+1 queries issues:

```
  ProjectMember Load (0.3ms)  SELECT "members".* FROM "members" WHERE
"members"."source_type" = $1 AND "members"."type" IN ('ProjectMember')
AND "members"."source_id" = $2 AND "members"."source_type" = $3 AND
"members"."type" IN ('ProjectMember') AND ("members"."requested_at" IS
NOT NULL)  ORDER BY "members"."id" DESC  [["source_type", "Project"],
["source_id", 8], ["source_type", "Project"]]
  User Load (0.1ms)  SELECT  "users".* FROM "users" WHERE "users"."id" =
$1  ORDER BY "users"."id" DESC LIMIT 1  [["id", 24]]
  User Load (0.1ms)  SELECT  "users".* FROM "users" WHERE "users"."id" =
$1  ORDER BY "users"."id" DESC LIMIT 1  [["id", 23]]
  ActiveRecord::SchemaMigration Load (0.2ms)  SELECT
"schema_migrations".* FROM "schema_migrations"
  ProjectMember Load (0.1ms)  SELECT  "members".* FROM "members" WHERE
"members"."source_type" = $1 AND "members"."type" IN ('ProjectMember')
AND "members"."source_id" = $2 AND "members"."source_type" = $3 AND
"members"."type" IN ('ProjectMember') AND ("members"."requested_at" IS
NOT NULL) AND "members"."user_id" = $4  ORDER BY "members"."id" DESC
LIMIT 1  [["source_type", "Project"], ["source_id", 8], ["source_type",
"Project"], ["user_id", 24]]
  ProjectMember Load (0.2ms)  SELECT  "members".* FROM "members" WHERE
"members"."source_type" = $1 AND "members"."type" IN ('ProjectMember')
AND "members"."source_id" = $2 AND "members"."source_type" = $3 AND
"members"."type" IN ('ProjectMember') AND ("members"."requested_at" IS
NOT NULL) AND "members"."user_id" = $4  ORDER BY "members"."id" DESC
LIMIT 1  [["source_type", "Project"], ["source_id", 8], ["source_type",
"Project"], ["user_id", 23]]
```

Without the N+1 queries issues:

```
  ProjectMember Load (0.3ms)  SELECT  "members".* FROM "members" WHERE
"members"."source_type" = $1 AND "members"."type" IN ('ProjectMember')
AND "members"."source_id" = $2 AND "members"."source_type" = $3 AND
"members"."type" IN ('ProjectMember') AND ("members"."requested_at" IS
NOT NULL)  ORDER BY "members"."id" DESC LIMIT 20 OFFSET 0
[["source_type", "Project"], ["source_id", 8], ["source_type",
"Project"]]
  User Load (0.5ms)  SELECT "users".* FROM "users" WHERE "users"."id" IN
(24, 23)  ORDER BY "users"."id" DESC
```
Signed-off-by: Rémy Coutable <remy@rymai.me>

lib/api/access_requests.rb

@@ -18,10 +18,9 @@ module API
           source = find_source(source_type, params[:id])
           authorize_admin_source!(source_type, source)
 
-          access_requesters = source.requesters
-          users = Kaminari.paginate_array(access_requesters.map(&:user))
+          access_requesters = paginate(source.requesters.includes(:user))
 
-          present paginate(users), with: Entities::AccessRequester, source: source
+          present access_requesters.map(&:user), with: Entities::AccessRequester, access_requesters: access_requesters
         end
 
         # Request access to the group/project

lib/api/entities.rb

@@ -93,14 +93,14 @@ module API
 
     class Member < UserBasic
       expose :access_level do |user, options|
-        member = options[:member] || options[:source].members.find_by(user_id: user.id)
+        member = options[:member] || options[:members].find { |m| m.user_id == user.id }
         member.access_level
       end
     end
 
     class AccessRequester < UserBasic
       expose :requested_at do |user, options|
-        access_requester = options[:access_requester] || options[:source].requesters.find_by(user_id: user.id)
+        access_requester = options[:access_requester] || options[:access_requesters].find { |m| m.user_id == user.id }
         access_requester.requested_at
       end
     end

lib/api/members.rb

@@ -18,11 +18,11 @@ module API
         get ":id/members" do
           source = find_source(source_type, params[:id])
 
-          members = source.members
+          members = source.members.includes(:user)
           members = members.joins(:user).merge(User.search(params[:query])) if params[:query]
-          users = Kaminari.paginate_array(members.map(&:user))
+          members = paginate(members)
 
-          present paginate(users), with: Entities::Member, source: source
+          present members.map(&:user), with: Entities::Member, members: members
         end
 
         # Get a group/project member