Rename and deduplicate LegacyClonePanel and ClonePanel

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The following discussion from gitlab-ce!23366 should be addressed:

• @sliaquat started a discussion: (+1 comment)

  I'm on the fence for this but should we rename the new ClonePanel to CloneDropdown and keep LegacyClonePanel as ClonePanel. I'm ok if you decide to keep this as is.

There's also some duplication of code we could remove. And I don't think wait_for_push belongs there.