Fix N+1 query problem for users on members view.

For each member being rendered on
Projects::ProjectMembersController#index, there was a query to the users
table to retrieve the corresponding user:

```
SELECT "users".* FROM "users" WHERE "users"."id" = ? LIMIT 1
```

app/finders/members_finder.rb

@@ -19,7 +19,7 @@ class MembersFinder
 
       sql = distinct_on(union)
 
-      Member.from("(#{sql}) AS #{Member.table_name}")
+      Member.includes(:user).from("(#{sql}) AS #{Member.table_name}")
     else
       project_members
     end