MM-16258: Adds new API endpoint + (App & Client & Store) to retrieve … (#11176)

* 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.

web/params.go

@@ -68,6 +68,7 @@ type Params struct {
 	IncludeMemberCount     bool
 	NotAssociatedToGroup   string
 	ExcludeDefaultChannels bool
+	GroupIDs               string
 }
 
 func ParamsFromRequest(r *http.Request) *Params {
@@ -268,5 +269,7 @@ func ParamsFromRequest(r *http.Request) *Params {
 		params.ExcludeDefaultChannels = val
 	}
 
+	params.GroupIDs = query.Get("group_ids")
+
 	return params
 }