Use join_policy API instead of joinable

The API is now under
 /groups/$group_id/setting/m.join_policy

and expects a JSON blob of the shape

```json
{
  "m.join_policy": {
    "type": "invite"
  }
}
```

where "invite" could alternatively be "open".

synapse/groups/groups_server.py

@@ -207,20 +207,24 @@ class GroupsServerHandler(object):
         defer.returnValue({})
 
     @defer.inlineCallbacks
-    def set_group_joinable(self, group_id, requester_user_id, content):
-        """Sets whether a group is joinable without an invite or knock
+    def set_group_join_policy(self, group_id, requester_user_id, content):
+        """Sets the group join policy.
+
+        Currently supported policies are:
+         - "invite": an invite must be received and accepted in order to join.
+         - "open": anyone can join.
         """
         yield self.check_group_is_ours(
             group_id, requester_user_id, and_exists=True, and_is_admin=requester_user_id
         )
 
-        is_joinable = content.get('joinable')
-        if is_joinable is None:
+        join_policy = _parse_join_policy_from_contents(content)
+        if join_policy is None:
             raise SynapseError(
-                400, "No value specified for 'joinable'"
+                400, "No value specified for 'm.join_policy'"
             )
 
-        yield self.store.set_group_joinable(group_id, is_joinable=is_joinable)
+        yield self.store.set_group_join_policy(group_id, join_policy=join_policy)
 
         defer.returnValue({})
 
@@ -854,6 +858,31 @@ class GroupsServerHandler(object):
         })
 
 
+def _parse_join_policy_from_contents(content):
+    """Given a content for a request, return the specified join policy or None
+    """
+
+    join_policy_dict = content.get("m.join_policy")
+    if join_policy_dict:
+        return _parse_join_policy_dict(join_policy_dict)
+    else:
+        return None
+
+
+def _parse_join_policy_dict(join_policy_dict):
+    """Given a dict for the "m.join_policy" config return the join policy specified
+    """
+    join_policy_type = join_policy_dict.get("type")
+    if not join_policy_type:
+        return True
+
+    if join_policy_type not in ("invite", "open"):
+        raise SynapseError(
+            400, "Synapse only supports 'invite'/'open' join rule"
+        )
+    return join_policy_type
+
+
 def _parse_visibility_from_contents(content):
     """Given a content for a request parse out whether the entity should be
     public or not