Resolve "Disabling push mirror raises error 500"

What does this MR do?

This MR moves RemoteMirror#remove_remote to a background job since the operation can become lengthy as per: https://gitlab.com/gitlab-org/gitlab-ee/issues/4099#note_50548278

We also need to generate a dynamic ref_name (now being renamed to remote_name) because we might update the remote with the same ref_name which will result in the deletion of the new data as well.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #4099 (closed)