Support SSH URI when Mirroring repositories
### Problem to solve
When adding a repository mirror to "Mirroring repositories" configuration, Gitlab insists on using full standard URI like `ssh://git@gitlab.com/gitlab-org/gitlab.git`. However, when using SSH, Git uses shorter `git@gitlab.com:gitlab-org/gitlab.git` URI. It is annoying when copy&pasting URIs between repositories.
### Proposal
Gitlab should understand the short repository URIs used by Git so that we can copy&paste them from "Clone with SSH" field.

### Links / references
issue