Fix 4120: Repository name cannot contain '/' character anymore

[LucasG0]

Fix #4120.

Note the current error message displayed to the user might not be very friendly. I am not sure whether we are fine with this or if we want to display a clearer one.

Also note that as discussed with @ogenstad the long term fix for this issue might be to store a repository using its id instead of name attribute so / characters could not be considered as file path separators.

This PR currently targets stable but we might want to re-target it to 0.16.1 branch once created.

[ogenstad]

LGTM. While it would be good with a more human friendly error message I think this is something that relates to these regex constraints as a whole and it's not specifically tied to this PR. At least here the cure is better than the strange error we had before.

I'll also create an issue to track the transition to storing the repos on disk with "id" instead of "name", at that point we can remove this regex again.

Other than that I'm just not certain where we should merge this for now as it's so close to the 0.16.0 release or if this would go to the 0.16.1. I think either or is fine. We can sync that during the day. It would be nice to add a towncrier newsfragment stating that this fixes #4120.

[LucasG0]

Merging as discussed with @dgarros.