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.

@nick.thomas I think transforming it on the backend is preferable, so that setting it over the API works too. I don't think we necessarily need to store it SCP style.

mentioned in issue gitlab-com/support-forum#2391 (closed)

mentioned in issue #3312 (closed)

We are indeed getting quite a few issues about this.

Can we prioritise support for SCP URLs?

/cc @mydigitalself

changed milestone to %Next 3-6 months