Find pattern for removing options from project admin

[agjohnson]

Many of the project admin options should be moved to our .readthedocs.yml file. We'll want a pattern for deprecating the option in our admin. Perhaps we implement a feature flag type model that flags new users as not having access to the model field. I'm not sure what the model forms would look like in this case, but might have to be dynamic and on __init__ remove these fields?

Some options here:

• https://github.com/jsocol/django-waffle -- betaish currently. Recommended project, but gargoyle-yplan seems to have more development
• https://gargoyle-yplan.readthedocs.io/en/latest/index.html -- this is the maintained fork of the deprecated Gargoyle project from Discus

Neither project mentions ripping up forms to do feature flagging specifically.

Refs #2663

[agjohnson]

readthedocs.projects.models.Feature does this! Closing.