Merge Request edit: make breadcrumbs consistent

What does this MR do?

As described in #329063 (closed), the breadcrumbs in the Edit view for merge requests is not as expected.

It currently shows "Project > Merge Requests", with the last item linking to the URL that is currently loaded (/edit).

This seeks to change it to, for example, Projects > Merge Requests > !123, where the MR ID links to the view that displays that merge request.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?) (It this needed?)
• [ ] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?) FOSS only
• [ ] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.