MediaGoblin 0.12.0

This release resolves two significant issues in the Celery backend media
processing. The first was causing processed media to be marked as failed and the
second was inhibiting useful error messages. We've also resolved installation
issues caused by deprecated upstream code in the Werkzeug and jsonschema
libraries.

We've added provisional `gmg serve` and `gmg celery` commands to simplify
deployment. These commands may change in the future and are not yet recommended
in the deployment documentation. If your deployment is already running smoothly,
there's no reason to switch at this stage.

Please note that installation currently fails on Debian Testing (Bookworm) and
Arch Linux at `./configure`. These issues appear to be related to versions of
autoconf > 2.69. We currently recommend deployment on Debian Bullseye and
Fedora 33.

Upgrading:

For detailed instructions on installing or upgrading, see "upgrading" and
"deploying" documentations.

If you have any problems, please drop in to the #mediagoblin IRC chat, report an
issue on our issue tracker or drop us an email to mediagoblin-devel@gnu.org.

Improvements:

 - Improve usability of report handling page (Rodrigo Martins)
 - Remove `--system-site-packages` from `./configure` in docs and CI builds as it
   is forced anyway (Ben Sturmfels)
 - Added provisional `gmg serve` and `gmg celery` commands (Ben Sturmfels)
 - Switch Atom feeds from deprecated werkzeug.contrib.atom to feedgenerator,
   upgrade werkzeug (Ben Sturmfels)
 - Document that `gmg dbupdate` is required after some config changes (Rodrigo
   Martins)
 - Remove Debian 10 development Dockerfile (BenSturmfels)
 - Add Debian 11 development Dockerfile and CI build (Ben Sturmfels)
 - Fix/document Guix setup for 100% passing test suite (Ben Sturmfels)
 - Convert `setup.py` to `setup.cfg` (Ben Sturmfels)
 - Document re-installation of plugins during upgrade [#5611] (Ben Sturmfels)
 - Remove unused `extlib/flask-wtf` code (Ben Sturmfels)
 - Remove unused `translitcodec` dependency (Ben Sturmfels)
 - Remove references to previous `tinymce` JS dependency (Ben Sturmfels)
 - Remove plugin for decommissioned Mozilla Persona (Jgart)
 - Document running multiple MediaGoblin instances on one server (Ben Sturmfels)
 - Begin conversion from jQuery to vanilla JS (Ben Sturmfels)

Bug fixes:

- Fix images being marked as failed after Celery restart since 0.10.0 [#5608] (Ben Sturmfels)
- Fix app logging calls not showing in Celery logs (Ben Sturmfels)
- Fix test suite on Debian 11 (Ben Sturmfels)
- Remove reference to jsonschema.compat no longer available upstream (Marco Pessotto)