Add group to field list

Allows groups and repeat groups to be nodes under a field list

@ctsims Mobile supports this as of 2.16 correct?
Let's go all the way to *make it a standard feature flag* (instead of just feature-flag-like). I'm not liking the idea of inventing yet another way to enable/disable features in Vellum since we have two perfectly good ones with feature flags and the plugin system.

I might even go so far as to say we should refactor the plugin system to use the feature flag dict, but that's completely unrelated to this PR.