New /api/{list} endpoint with field selection and filtering

To support the new Admin UI, we need a new generic List API Endpoint:

GET /keystone/api/{list.path}

It should implement the following query parameters:

• search (string) search query applied to the List’s default search fields
• filters (JSON Object) used to filter the query; all top-level keys in this object that match field paths would be passed to each field’s addFilterToQuery(filter, query) method, see #1539
• select (space-delimited string of paths) used to select fields to be returned; defaults to all fields defined in the list.
• populate (space-delimited string of paths) relationship fields to populate
• sort (space-delimited string of paths) sorts the query; paths are passed to Query.sort as a space-separated list. Inverse sort is specified by prefixing - to any path.
• limit (number) the maximum number of items to return; default 100
• skip (number) the number of items to skip (used for pagination); default 0
• expandRelationshipFields (boolean) populates relationship fields with id and name properties (name retrieved with list.getDocumentName(item))

The data returned should be in the format:

{
  results: [Array of items],
  count: Number, // total results matching the filters
}

[jjhesk]

@JedWatson thanks!

[JedWatson]

See #2557