Follow-up from "Remove existence check from ProjectUrlConstrainer"

The following discussion from !19412 (merged) should be addressed:

• @engwan started a discussion: (+2 comments)

  Had to move this just before the Projects#show route below so this gets higher priority. Previously it was just failing on Projects#show with a non-existent project and then matching this.

  This is the same theme for the changes to the route files. Defining the more specific routes before the generic fallback.

TODO:

Add link to the issue #28848 (closed) to project legacy routes. Update milestone to remove the old code