This is a follow up of historical [bug 237747].

I have no idea how Launchpad (LP) was structured back then, but nowadays
Launchpad is the place for various (upstream) projects, Ubuntu being
just one of them.  As I think it is useful to link RHBZ bugs to upstream
ones, so to make it easier for LP ones in particular, I suggest
these changes:

"Add External Bug: Tracker" dropdown menu:

- https://bugs.launchpad.net/ubuntu
+ https://bugs.launchpad.net/bugs

- Ubuntu Launchpad
+ Launchpad

(or "Launchpad (Ubuntu and other projects)" if we want to ease referring
to Ubuntu side)

Actually added Launchpad tracker

- https://bugs.launchpad.net/ubuntu/+bug/<id>
+ https://bugs.launchpad.net/bugs/<id>


Note that the current state allows to link LP bugs for other projects
as well because IDs are unique globally per LP and "/ubuntu/+bug/<id>"
URL will get redirected to "/<respective-project>/+bug/<id>" when
appropriate, _but_ the current labelling is counter-intuitive
(e.g., I rather used a comment to link Zope bug: [bug 878961 comment 4]).