Until now we had two regex/extensions for the cross-project links. One
for forks one for regular project but this lead to the situation where
if you have a pattern such as /# the first part
() would be linked to the fork of the project of that
user and the second part (#) would link to the ticket/PR
with this id in the original project.
That was because the regex for cross-link project was matching the
content of the link generated by the cross-link fork.
By moving to one regex for all, we fix this situation.

We're also adding unit-tests for this part of the code to help with
maintenance and stability.