When querying merge requests via GraphQL and querying the `mergeable`
or `detailedMergeStatus` fields, it can result to N+1 SQL queries
when checking for the approval state of the MR.

This removes the N+1 queries problem by preloading associations
that are being used in `MergeRequests::Mergeability::CheckApprovedService`.

Changelog: performance
EE: true