auth/utils: Fix circular import triggered by sphinx autodoc
Fix "cannot import name 'ADD_FORGE_MODERATOR_PERMISSION' from 'swh.web.auth.utils'" when autodoc processes the admin_views module from swh.web.add_forge_now.
https://jenkins.softwareheritage.org/job/DDOC/job/dev/2156/artifact/swh-docs/docs/errors.log
Also remove no longer needed swh/web/common folder.
Migrated from D8290 (view on Phabricator)