Gitlab repository integration

I've added a basic Gitlab integration. I think I covered the same functionality as with Github:

* shows up in the application list
* shows up in the application's show action
* links files in backtraces

Note that I'm not sure if this works with older versions of Gitlab because as far as I know they've changed the URL format from ../tree/.. to ../blob/.. like on Github.

I hope the test is acceptable but I'm open for suggestions :-)