This requires the Can override approvals in merge request
option checked in repository settings.

This update also includes:

Adding some relationship links:

* ApprovalMergeRequestRule has_one Project
* ApprovalProjectRule has_many ApprovalMergeRequestRules
* ApprovalProjectRule has_many ApprovalMergeRequestRuleSources
* EE::MergeRequest has_many ApprovalMergeRequestRuleSources
* EE::MergeRequest has_many ApprovalProjectRules
* Scopes for Approval Merge Requests
* Scopes for EE:MergeRequest
* Two memoized functions for maximum/minimum IDs in the EE usage ping