Ensure consistent contexts across controllers #1845

Most endpoints default to context=view, but GET /wp/v2/users/<id> defaults to embed: 2d4cdbc

Some endpoints only have view, the meta endpoint only has edit, and other endpoints have edit.

For a client trying to making requests, these inconsistencies are really confusing. We should make sure we have consistent default contexts across controllers, and each controller supports similar context names.

Related #1797, #1796

Rediscovered in #1838

Previously #702