Support scp-style URLs in pull (and push?) mirroring

The following discussion from !2551 (merged) should be addressed:

• @DouweM started a discussion: (+4 comments)

  How hard would it be to support the git@gitlab.com:gitlab-org/gitlab-ee.git syntax too? I think that if we don't support that, we'll get a lot of support requests about "invalid URL" errors :)

We can transform the URL to be well-formed in the frontend or backend and store as ssh://..., or we can accept scp-style URLs and store them as-is.

The URLs end up being written to a .git/config file as we set up a remote for each mirror, so either approach would work.