Security: Improve overall security in a few ways (#12501)

- Make sure we're always using regex with timeouts for user controlled data
- Adds rate limiting to the token endpoint (configurable)
- Signs the classifier pickle file with the SECRET_KEY and refuse to load one which doesn't verify.
- Require the user to set a secret key, instead of falling back to our old hard coded one

Dockerfile

@@ -237,8 +237,8 @@ RUN set -eux \
   && echo "Adjusting all permissions" \
     && chown --from root:root --changes --recursive paperless:paperless /usr/src/paperless \
   && echo "Collecting static files" \
-    && s6-setuidgid paperless python3 manage.py collectstatic --clear --no-input --link \
-    && s6-setuidgid paperless python3 manage.py compilemessages \
+    && PAPERLESS_SECRET_KEY=build-time-dummy s6-setuidgid paperless python3 manage.py collectstatic --clear --no-input --link \
+    && PAPERLESS_SECRET_KEY=build-time-dummy s6-setuidgid paperless python3 manage.py compilemessages \
     && /usr/local/bin/deduplicate.py --verbose /usr/src/paperless/static/
 
 VOLUME ["/usr/src/paperless/data", \