Refactor reference regexes and identity scopes

What does this MR do?

This MR makes some changes to the Identity model and Mentionable::ReferenceRegexes to make it easier to extend this code in EE. This is necessary for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8198 as otherwise we can not move prepend to the end of the file, which in turn means we can't reduce the chance of running into merge conflicts for these files.

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8198

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides