Generate guest access token on 3pid invites

This means that following the same link across multiple sessions or
devices can re-use the same guest account.

Note that this is somewhat of an abuse vector; we can't throw up
captchas on this flow, so this is a way of registering ephemeral
accounts for spam, whose sign-up we don't rate limit.

synapse/storage/schema/delta/30/threepid_guest_access_tokens.sql

@@ -0,0 +1,24 @@
+/* Copyright 2016 OpenMarket Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+-- Stores guest account access tokens generated for unbound 3pids.
+CREATE TABLE threepid_guest_access_tokens(
+    medium TEXT, -- The medium of the 3pid. Must be "email".
+    address TEXT, -- The 3pid address.
+    guest_access_token TEXT, -- The access token for a guest user for this 3pid.
+    first_inviter TEXT -- User ID of the first user to invite this 3pid to a room.
+);
+
+CREATE UNIQUE INDEX threepid_guest_access_tokens_index ON threepid_guest_access_tokens(medium, address);