REST API: List users by capability or level for author selection

[aduth]

Related: #2100 (specifically #2100 (comment))

The author selection should not present options for users without the edit_posts capability. In default installations, this means excluding the Subscriber role. The users listing endpoint currently supports filtering by role. However, we ought not to hard-code default roles since plugins or themes can add their own custom roles. Therefore, we should seek to enhance the user listing endpoint with one of a few options:

• Support filtering by capability
• Support filtering by authors type
• Support filtering by user_level
  • authors is equivalent to user_level != 0

The latter two mimic the behavior of the wp_dropdown_users function used in the current editor to display user options. The who values is an undocumented parameter supported by the WP_User_Query class which filters by user_level != 0.

References:

• post_author_meta_box
• wp_dropdown_users
• WP_User_Query

[aduth]

Noting that while not ideal, it is possible to filter this information client-side: When requesting with ?context=edit query parameter, each user entry in the response will include a capabilities object.