Can remove membership

lib/helpers/groups_helper.dart

@@ -100,6 +100,13 @@ class GroupsHelper {
           .map((f) => cast<int>(f))
           .toSet();
 
+  /// Remove group membership
+  Future<bool> removeMembership(int groupID) async =>
+      (await (APIRequest(uri: "groups/remove_membership", needLogin: true)
+                ..addInt("id", groupID))
+              .exec())
+          .isOK;
+
   /// Turn an API entry into a group object
   Group _getGroupFromAPI(Map<String, dynamic> map) {
     return Group(