Support links to other issue trackers in Bitbucket specific issue strings (e.g., #123). (BB-6912)

Currently custom regex links that try to link (#\d+) to other issue trackers are blocked - e.g., trying to link BCLOUD-123 to Redmine.

This would be understandable when JIRA integration or the BB Issue Tracker are enabled for that repo, but there's zero reason to block these types of regex when those are disabled - it smacks of an anti-competitive restriction. I hope this restriction can be lifted - thanks.