Try harder to open GitHub links

Links that can't be opened by the app will try to get as close as
possible.

For example, if the app can't open
https://github.com/github/android/issues it will try to open
https://github.com/github/android/ and then https://github.com/github/
Remove the UrlLauncher class and treat all URIs in the UriLauncherActivity class
Remove tests that use the removed UrlLauncher
Merge pull request #613 from jonanp/links

Add alternative when failed opening some links
Revert "Revert "Add forking support""
Merge pull request #626 from github/revert-614-revert-612-fork

Add possibility to fork a repository