Enforce .git in the URL when importing Repo by URL

Problem

If a user importing a Git repo using the Repo by URL importer omits the .git extension in the URL, the import will get stuck.

This issue is exacerbated by the fact that, according to the discussion in this MR, even GitLab no longer redirects git requests without .git.

Proposed solution

We need to enforce that the URL ends with .git. To do that, we should add a validation to the URL field that would fail (and disable the next action) unless the URL ends in .git upon clicking on the Create project button.