Add joinability for groups

Adds API to set the 'joinable' flag, and corresponding flag in the
table.

synapse/storage/group_server.py

@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
 # Copyright 2017 Vector Creations Ltd
+# Copyright 2018 New Vector Ltd
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -29,6 +30,18 @@ _DEFAULT_ROLE_ID = ""
 
 
 class GroupServerStore(SQLBaseStore):
+    def set_group_joinable(self, group_id, is_joinable):
+        return self._simple_update_one(
+            table="groups",
+            keyvalues={
+                "group_id": group_id,
+            },
+            updatevalues={
+                "is_joinable": is_joinable,
+            },
+            desc="set_group_joinable",
+        )
+
     def get_group(self, group_id):
         return self._simple_select_one(
             table="groups",