Adds back missing code to enable the mirror repository checkbox

What does this MR do?

Adds back missing code to enable the mirror repository checkbox

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • [ ] Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• 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 #3146 (closed)

@stanhu pipeline is still running, can you review this or should I ask a frontend maintainer? (Only adds code back )

@filipa Code looks good to me, but please ask a frontend maintainer to do a final review.

Also, is there a way we can add a test for this?

@stanhu thanks, makes sense.

Will look into rspec to add one!

added 1 commit

• faa40419 - Adds tests for mirror repository checkbox

Compare with previous version

The failing job is a transient failure from master: https://gitlab.com/gitlab-org/gitlab-ce/issues/36150

@iamphill can you please review this one? Thanks!

assigned to @iamphill

approved this merge request

enabled an automatic merge when the pipeline for faa40419 succeeds

merged

mentioned in commit d8736b89

mentioned in commit e37f8973

Picked in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2680, will merge into 9-5-stable-ee ready for 9.5 RC3

removed Pick into Stable label