Model changes for code owner approval rules

What does this MR do?

This is a first step in https://gitlab.com/gitlab-org/gitlab-ee/issues/4418 not all code is used in this MR, but it will be in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9293.

This adds a partial unique index for approval_merge_request_rules, the name must be unique scoped to a merge request when code_owners = 't'. This is also enforced by validations. A helper method for creating/finding code owner rules avoiding race conditions.

This index is safe to add, as we already only create one code_owner row in that table. Now the database will enforce this.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer