Merge two of the room join codepaths

There's at least one more to merge in.

Side-effects:
 * Stop reporting None as displayname and avatar_url in some cases
 * Joining a room by alias populates guest-ness in join event
 * Remove unspec'd PUT version of /join/<room_id_or_alias> which has not
   been called on matrix.org according to logs
 * Stop recording access_token_id on /join/room_id - currently we don't
   record it on /join/room_alias; I can try to thread it through at some
   point.

synapse/rest/client/v1/room.py

@@ -216,11 +216,7 @@ class RoomSendEventRestServlet(ClientV1RestServlet):
 
 # TODO: Needs unit testing for room ID + alias joins
 class JoinRoomAliasServlet(ClientV1RestServlet):
-
-    def register(self, http_server):
-        # /join/$room_identifier[/$txn_id]
-        PATTERNS = ("/join/(?P<room_identifier>[^/]*)")
-        register_txn_path(self, PATTERNS, http_server)
+    PATTERNS = client_path_patterns("/join/(?P<room_identifier>[^/]*)$")
 
     @defer.inlineCallbacks
     def on_POST(self, request, room_identifier, txn_id=None):
@@ -229,60 +225,22 @@ class JoinRoomAliasServlet(ClientV1RestServlet):
             allow_guest=True,
         )
 
-        # the identifier could be a room alias or a room id. Try one then the
-        # other if it fails to parse, without swallowing other valid
-        # SynapseErrors.
+        handler = self.handlers.room_member_handler
 
-        identifier = None
-        is_room_alias = False
-        try:
-            identifier = RoomAlias.from_string(room_identifier)
-            is_room_alias = True
-        except SynapseError:
-            identifier = RoomID.from_string(room_identifier)
+        room_id = None
+        hosts = []
+        if RoomAlias.is_valid(room_identifier):
+            room_alias = RoomAlias.from_string(room_identifier)
+            room_id, hosts = yield handler.lookup_room_alias(room_alias)
+        else:
+            room_id = RoomID.from_string(room_identifier).to_string()
 
         # TODO: Support for specifying the home server to join with?
 
-        if is_room_alias:
-            handler = self.handlers.room_member_handler
-            ret_dict = yield handler.join_room_alias(
-                requester.user,
-                identifier,
-            )
-            defer.returnValue((200, ret_dict))
-        else:  # room id
-            msg_handler = self.handlers.message_handler
-            content = {"membership": Membership.JOIN}
-            if requester.is_guest:
-                content["kind"] = "guest"
-            yield msg_handler.create_and_send_event(
-                {
-                    "type": EventTypes.Member,
-                    "content": content,
-                    "room_id": identifier.to_string(),
-                    "sender": requester.user.to_string(),
-                    "state_key": requester.user.to_string(),
-                },
-                token_id=requester.access_token_id,
-                txn_id=txn_id,
-                is_guest=requester.is_guest,
-            )
-
-            defer.returnValue((200, {"room_id": identifier.to_string()}))
-
-    @defer.inlineCallbacks
-    def on_PUT(self, request, room_identifier, txn_id):
-        try:
-            defer.returnValue(
-                self.txns.get_client_transaction(request, txn_id)
-            )
-        except KeyError:
-            pass
-
-        response = yield self.on_POST(request, room_identifier, txn_id)
-
-        self.txns.store_client_transaction(request, txn_id, response)
-        defer.returnValue(response)
+        yield handler.do_join(
+            requester, room_id, hosts=hosts
+        )
+        defer.returnValue((200, {"room_id": room_id}))
 
 
 # TODO: Needs unit testing