Optimize query in Groups::AcceptingProjectCreationsFinder class

When we render the 'new projects' page (/projects/new), we retrieve a list of groups in which the current user is allowed to create a project. This list is coming from Groups::AcceptingProjectCreationsFinder class.

In this MR, we found that the query could be optimized: one of the indexes we use can be improved.

Some findings from the query plan:

• The problematic part is Rows Removed by Filter: 37 (this is for user_id 1), there are multiple sub queries
• The query is consulting members table and filtering on: user_id, source_type, access_level, type, requested_at
• It is using the indexes index_members_on_user_id_and_access_level_requested_at_is_null and idx_members_on_user_and_source_and_source_type_and_member_role but it still results in too many rows

I tried adding an index on user_id, access_level, source_type, type but this did not help for the subqueries that use members.access_level IN (30,40,50)

Definition of done

• Optimize the query so we do not see Rows Removed by Filter: 37 anymore (or larger number)
• Investigate if we have any redundant indexes that could be replaced by a better index