Extract Feature Flag Details

[tn3rb]

Currently, Feature Flags (FFs) are defined in Core, but the FF admin is part of the Barista EE plugin. The FF form options were also defined in the Barista plugin. This is awkward because whenever a new FF is added, you need to add the FF to core as well as update the Barista plugin to add the new FF to the FF admin page form.

This PR:

• extracts the FF form options from the Barista plugin
• the Cafe branch MOD/CORE/feature-flags-form-options adds the FF form options to a new FeatureFlag class

So now, we only need to add new FFs to core, and the FF admin will just work without any further modifications

[alexkuc]

FYI, E2E fails due to this https://github.com/eventespresso/barista/issues/1281

Edit: to test PRs against E2E need to manually trigger them from Actions specifying the Cafe's branch as per the workaround above