Update mirror user for all projects deleted user is mirror_user for

What does this MR do?

Update mirror user for all projects that the user being deleted is mirror_user for. One of the project's group's owner becomes the new mirror_user.

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

• Should we skip projects where project creator and mirror user are the same user?
• I thought about making this yet another server under projects, but it felt overkill.

Why was this MR needed?

If the mirror user is not updated, the project is left with a stale value for mirror_user_id and that causes calling save on the project to fail.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2532 (closed)

/cc @lbot @MrChrisW @stanhu