Add REST API endpoint

Fieldmanager should add a custom REST API endpoint that allows an application to query field information for a context/subcontext. This would allow apps like [Calypso](https://github.com/Automattic/wp-calypso), [mobile apps](https://wordpress.org/mobile/), etc., to leverage field data to render an editing interface for Fieldmanager fields.

At a high level, we'd need to:
- [ ] Add a GET endpoint to surface base field data (field type, label, description, etc.) for all fields added to a given context and subcontext (e.g. term/category or post/post)
- [ ] Sanitize and save data on create and update requests for posts, user, and terms
- [ ] Add endpoints for submenu contexts