Release 2256

Trello card

https://trello.com/c/ql47nFtH

Context

1. Update Feature class to read from a JSON config: We currently have a couple of implementations of feature flags in the app (the Feature class and Flipper).
   They aren't currently very visible, which means there are features that we don't know about (capped_bookings), or features that we though were turned on and actually weren't (sending reminder emails). We investigated using the Flipper dashboard for visibility, but while there is a feature (Flipper cloud) where you can see flags across all environments, it's a paid feature.
   Instead, we can use a JSON config file to manage feature flags. We will read this into a dashboard which will be accessed from any environment. This also removes the subject_specific_dates flag which was enabled in all environments, and replaces the phase var (This was an earlier iteration of feature flagging, currently only used to disable candidate dashboard access) with feature flags instead.
2. Add feature flag dashboard: We want to be able to see all of our feature flags at a glance, so that we can monitor them across all environments.
3. Remove Flipper: Flipper has been used previously for feature flags. However, it doesn't allow us to see features across all environments, so we are using a custom implementation instead.

Guidance to review

Visit /feature_flags for the "dashboard". Is this an improvement? Does this need to be behind auth, Its all visible anyway 🤷?