WIP: Find projects for merge request

What does this MR do?

When creating a merge request, the current users projects are checked to see if any of them are forks of the project that the merge request is being opened on. If so, they are added to the source project list.

Code isn't quite working yet, the users projects are found but the list of branches to pick from isn't updated (I suspect this will have to be implemented from scratch as it looks like the first drop down has done nothing until now)

Are there points in the code the reviewer needs to double check?

I will refactor the query for finding projects to make it better but after that it should probably be checked for any obvious speed issues

Why was this MR needed?

Fixes #17722 (moved)

Screenshots (if relevant)

Second option was found using the new code.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• Conform by the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?