* MM-16258: Adds new API endpoint + (App & Client & Store) to retrieve Users who would be removed from a list of hypothetical group IDs representing the synced groups.
* MM-16258: Adds roles to JSON response.
* MM-16258: Updates GetByIDs to use Squirrel.
* MM-16258: Puts as much as possible into Squirrel.
* MM-16258: Changes names of methods, functions, and route.
* MM-16258: Updates some comments.
* MM-16258: Extra validation of group_ids parameter.
* MM-16258: Changes validation of group_ids query param.
* MM-16258: Rename a variable and a constant.
* MM-16258: Fix test.