Add feature flag support via Flipper

[mwtrew]

Status

• Closes #1101
• Related to https://github.com/RaspberryPiFoundation/editor-standalone/pull/723

Points for consideration:

Security

• Adds a new admin panel at /admin/flipper. This uses a route constraint to deny access to non-admin users (based on session cookie).
• Disabled feature flags are not shared with the client. However, at present all enabled features are shared with the client, regardless of whether they're expected to have any effect on the client.

Performance

• In this implementation, each School is a potential Actor (in Flipper terms). Flipper prefers to load feature data once per request and perform membership checks in memory to reduce the number of calls to the DB, but this can be less efficient if there are many Actors. Therefore, by default, there is a limit of 100 Actors per feature. See the Flipper docs - Actors.
• See also Flipper docs - Performance

What's changed?

• Adds Flipper UI, allowing admin users to manage feature flags, and an endpoint that exposes enabled feature flags to the client.

Steps to perform after deploying to production

• Migration(?)