Support gitlab subgroups

This commit updates org-sync-gitlab-backend to allow it to properly parse urls with multiple subgroups. I've replaced the regex that was used, to now use the url-parse functions that come with emacs.

@gjnoonan @plajjan Can confirm this bug and confirm the fix works with latest gitlab.

Can you add some example URLs that show what a URL with subgroups might look like? I imagine it will be easier for the next person coming around, wondering what this function does, to see some example inputs. A proper test case would of course be best but seeing how we have a long way to go there (there are like no tests now) I think adding a comment above the function with a few examples is the second best thing.

@plajjan sure! The git ssh address I'm using is: [email protected]:digital/ops/test.git

So in this case the digital is the first group, ops is the second and test is the project name.