Similar to the existing `role`/`role__in`/`role__not_in` query arguments, this adds support for three new query arguments in `WP_User_Query`:
* `capability`
* `capability__in`
* `capability__not_in`
These can be used to fetch users with (or without) a specific set of capabilities, for example to get all users
with the capability to edit a certain post type.
Under the hood, this will check all existing roles on the site and perform a `LIKE` query against the `capabilities` user meta field to find:
* all users with a role that has this capability
* all users with the capability being assigned directly
Note: In WordPress, not all capabilities are stored in the database. Capabilities can also be modified using filters like `map_meta_cap`. These new query arguments do NOT work for such capabilities.
The prime use case for capability queries is to get all "authors", i.e. users with the capability to edit a certain post type.
Until now, `'who' => 'authors'` was used for this, which relies on user levels. However, user levels were deprecated a long time ago and thus never added to custom roles. This led to constant frustration due to users with custom roles missing from places like author dropdowns.
This updates any usage of `'who' => 'authors'` in core to use capability queries instead.
Subsequently, `'who' => 'authors'` queries are being **deprecated** in favor of these new query arguments.
Also adds a new `capabilities` parameter (mapping to `capability__in` in `WP_User_Query`) to the REST API users controller.
Also updates `twentyfourteen_list_authors()` in Twenty Fourteen to make use of this new functionality, adding a new `twentyfourteen_list_authors_query_args` filter to make it easier to override this behavior.
Props scribu, lgladdly, boonebgorges, spacedmonkey, peterwilsoncc, SergeyBiryukov, swissspidy.
Fixes#16841.
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