metainfo, gitlab: Update and add URLs
Initiatives#51 proposes to update and unify links in metainfo for GNOME apps. We were using the old wiki link for translation and did not use the welcome.gnome.org page for contributors.
Fix and add these links.
Closes #2938 (closed)
Also updates a link in the Bug.md
issue template.
Edited by Matthijs Velsink