Update bug URLs
After the Bugzilla to GitLab migration, some links redirect to strange places.
For the "new bug" links (was http://bugzilla.gnome.org/enter_bug.cgi?product=Pan), I think it's better to point to https://gitlab.gnome.org/GNOME/pan/issues instead of https://gitlab.gnome.org/GNOME/pan/issues/new, so people can first check if the bug already exists. Also, people that are not logged in would be redirected to the sign-in page, which could be confusing.
For existing Bugzilla bugs, change the URLs to https://