Draft: Features editor

Summary

Fixes #1189

When complete, will allow configuring instance features through a UI, overriding them on an account and/or instance level.

Overriding features does not make them available by the backend, obviously. But this can be used to fix problems with the automatic feature detection, and may be used to disable features in the UI.

TODO

• Make getFeatures() accept state (instead of instance).
• Merge features from Soapbox Config into getFeatures.
• ???

Screenshots (if appropriate):