Turn "depends" and "blocks" fields into comments

In lieu of actual issue dependency, we can iterate over every value in the Bugzilla Depends and Blocks fields and craft a GitLab comment containing both, e.g.

• Depends
• https://bugzilla.gnome.org/show_bug.cgi?id=...
• https://bugzilla.gnome.org/show_bug.cgi?id=...
• https://bugzilla.gnome.org/show_bug.cgi?id=...
• Blocks
• https://bugzilla.gnome.org/show_bug.cgi?id=...
• https://bugzilla.gnome.org/show_bug.cgi?id=...

This would be added as the last comment in a migration.