Create a bugzilla-migration user

The script migrates issues and comments as the user account whose access token is given. That means the maintainer who runs the script is credited for all the issues (and gets subscribed to all of them)

We discussed on IRC creating a "bugzilla-migration" user. A GitLab admin could then hand out a time-limited access token to whatever maintainer wanted to migrate their Bugzilla issues.

For testing purposes it would also be nice to have the same user on gitlab-test.gnome.org, perhaps with a publicly available access token.

It also seems possible from the GitLab API to post comments as another user if the authenticated account has admin privileges. Since we can already match email addresses to GitLab users in the script, it seems possible to do something like issue.save(sudo='ptomato') to post an issue as me, for example.