ui/features: Environment-configurable feature flags with UI overrides

[ssangervasi]

Closes [The hole in my soul]

Code Changes

• Check out that readme!
• The features slice now has persisted has state that can save flag changes for the user
• The FeaturesPanel can toggle (and reset) flags from within the UI. Access tucked under the user menu.
• Use NEXT_PUBLIC_APP_ENV for app-specific environment config - Next.js discussion
• Removed react-feature-flags dependency

Steps to Confirm

• Check out the loom below for switching flags when the build environment changes
• Check out the gif for how to toggle a feature (navV2) that's currently dev-only
• Try adding a new flag and see if its convenient

Pre-Merge Checklist

• All CI Pipelines Succeeded
• Documentation:
  • documentation complete, PR opened in fidesdocs
  • documentation issue created in fidesdocs
• Issue Requirements are Met
• Relevant Follow-Up Issues Created
• Update CHANGELOG.md

Description Of Changes

Lovely

Readme update:

Defaults switching per build environment
https://www.loom.com/share/28a23c54fcf2415299d8bfa92eb3d61c

[allisonking]

amaaaaaazing 🎏

[ssangervasi]

Pro-tip: I'm leaving this here without the flag check so that those who know the trick can access the panel in prod builds even when the menu entry isn't shown.

[ssangervasi]

Here's a screenshot with the additional message, showing a flag toggled on that would not be on by default (in env "test"):