Migrate swh-web production database from SQLite to PostgreSQL

Now that the "Save code now" service gets more and more used, we started to encounter some SQLite concurrency issue, see #2891 (closed) and sentry related issue.

This calls for the use of a better Django database backend in production, PostgreSQL being the obvious choice.

This task will track the actions that need to be done to perform that migration process, notably:

• the creation of a swh-web database in a PostgreSQL server somewhere in swh staging / production environment
• the adaptation of Django production settings and swh-web configuration to use that database
• the dump of current swh-web SQLite database content to JSON
• the load from those JSON data into the new swh-web PostgreSQL database

Plan or action so far:

• D5392: Migrate swh.web settings to declare the use of postgres
• D5391: Ensure migration tools and setup works in docker
• infra/puppet/puppet-swh-site!336: staging: Prepare db (puppet)
• production: Deactivate puppet on moma, webapp1
• infra/puppet/puppet-swh-site!337: staging: Prepare swh-site setup change to webapps
• Land, tag package swh-web (so the migration tools is available on nodes to migrate)
• Check everything is fine in webapp.staging.swh.network

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• production: Prepare swh-site setup change to webapps
• production: Prepare new swh-web db (manual creation)
• production: Run swh-web upgrade and migrate previous data dump on new db
• production: Run upgrade on moma (reactivate puppet)

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Migrated from T2945 (view on Phabricator)