Improve UX when renaming a project

Expected behavior

When renaming a project, GitLab should properly check if the name the user is attempting to rename to is already taken. If the new name already exists the user should be alerted with an error message and no move operation should occur.

Observed behavior

Customer reports that in version 7.12 the project they tried to rename on top of an existing project (accidentally) went into oblivion after the rename operation. I was unable to reproduce this issue on either 7.12 or 8.0. However, I do see that there is no error message telling a user what happened. I believe that if we improve this checking process and show an error that any bug the user experienced will likely be prevented in the future.

One interesting thing I notice when I tested this is that although the new name already exists, when I attempt to the rename the project the page reloads at the same URL, but the project's name field has been altered to the new name. Also, the left navigation now links to the new project name even though the move never happened.

Steps to reproduce this behavior:

  1. Create or find 2 test projects in the same namespace.
  2. Go to settings for one project.
  3. Rename the project to the other existing project by changing both the name and path (changing name is not necessary, behavior still happens if you only change path).
  4. Observe that the URL path is the same. Hover over left nav items and observe they point at the new name even though the project never moved. No error message is produced in UI or in logs (that I could find).

Next steps

  • How can we improve the user experience?
  • How can we improve error checking to ensure no project is lost during a rename?

/cc @JobV @dzaporozhets