Move waffle switches into the admin

[stevejalim]

Description

Now that we're using a centralised DB again, we can move our waffle-switch approach away from a separate repo (the waffle_configs directory in momeao/www-config and instead use the Django admin on the CMS deployment to manage them.

The admin on the CMS deployment is connected to the primary DB, and waffle config changes will be flowed down to the replicas for the web pods effectively immediately, which will be faster than the current process, which reads from www-config a most every five minutes.

Whether we go (back?) to https://waffle.readthedocs.io/en/stable/ or use something else, we'll need to decide on an approach and also migrate existing switches from www-config into the Bedrock primary DB for Dev, Stage and Prod (as these all appear to have waffle configs in www-config.

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Success Criteria

• [ ] Feature flipping via the Django admin is enabled
• [ ] The latest state of www-config's waffle_configs dir is migrated into the DB
• [ ] www-config is no longer read from as part of ./bin/run-db-update.sh
• [ ] waffle_configs dir is removed from www-config
• [ ] Documentation is updated accordingly

[janbrasna]

@stevejalim FYI I've been cleaning up the switches to only keep the ones in use for #14092 — so after https://github.com/mozmeao/www-config/pull/582 this should be all — and only those used live will remain, not having to migrate junk.

[stevejalim]

Thanks @janbrasna - FYI @robhudson