Resolve "Mirroring interface improvements"

What does this MR do?

Consolidates push and pull mirror interface into a single panel.

CE: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20837/diffs

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

Why was this MR needed?

push and pull mirror interfaces are very similar

Screenshots (if relevant)

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(the ui is slightly different to these gifs below, as seen above, but keeping these gifs as they demonstrate function that hasn't changed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • 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
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #3959 (closed)