Feature flag system

[brainwane]

What's the problem this feature will solve?
It's annoying to roll out changes to Test PyPI and not PyPI.org, or to only certain users -- we're running into that in #5661 (comment) . For instance, we can't currently change configuration to indicate "this should be enabled for all users on test.pypi.org, including new users created after we flip the switch, but only per-individual on pypi.org".

Describe the solution you'd like
A feature flag system.

• Most important is the ability to, either via a config file or a GUI interface for PyPI admins, enable a feature for all users on test.pypi.org, including new users created after we flip the switch, but not have that affect default users on pypi.org.
• Nice-to-have: a way for users to opt in to beta test new features in their account settings, so we don't have to create and run separate Google forms and similar mechanisms for beta periods.

[di]

Bumping this. I'd like us to experiment with using a standalone feature flagging service, and https://flagsmith.com/ looks like a good option:

• we can stand it up ourselves for free
• it's open-source
• it's Python

Key features we'll want to check for when evaluating this:

• ability to set feature flags per-user
• ability to segment users based on some arbitrary characteristics (e.g. what domain they have verified emails for, whether they have 2FA enabled, etc)
• ability to fail-closed in a reasonable way

Steps towards resolving this:

• prototype using this in local development by adding it to our docker compose services
• stand up an an instance in our staging environment
• deploy this to staging
• stand up an an instance in our production environment
• deploy this to production