Allow bugs to link to external branches

[Alberto Mardegan]

Launchpad has a feature where you can add a link to a branch to a bug report. This is helpful in that it gives the bug submitter (or whoever is also affected by the bug) a chance to review the branch.

Some projects (such as snapd) use Launchpad as a bug tracker, but have their branches in github. For such cases, it would be nice if the UI for "link to a branch" accepted absolute URLs and just added them to the bug without further processing (of course, in the future someone will ask to track the status of that and show whether the branch has been merged :-) ).

Or maybe this could be viewed more generally, that is allow people (maybe only the project owners, to avoid spamming) to add any links to a bug.