Merge branch 'backport-part-of-ee-744' into 'master'

Make Member#add_user set access_level for requesters

## What does this MR do?

This is a backport of https://gitlab.com/gitlab-org/gitlab-ee/commit/c902d8d48409a4d46f7669fc5fc85b9ba86e71d6 from gitlab-org/gitlab-ee!744.

## Why was this MR needed?

This change is only used in EE right now, but there's no reason why it wouldn't be part of CE.

See merge request !6649

app/models/member.rb

@@ -103,7 +103,12 @@ class Member < ActiveRecord::Base
       }
 
       if member.request?
-        ::Members::ApproveAccessRequestService.new(source, current_user, id: member.id).execute
+        ::Members::ApproveAccessRequestService.new(
+          source,
+          current_user,
+          id: member.id,
+          access_level: access_level
+        ).execute
       else
         member.save
       end