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/registration.py

@@ -387,3 +387,47 @@ class RegistrationStore(SQLBaseStore):
             "find_next_generated_user_id",
             _find_next_generated_user_id
         )))
+
+    @defer.inlineCallbacks
+    def get_3pid_guest_access_token(self, medium, address):
+        ret = yield self._simple_select_one(
+            "threepid_guest_access_tokens",
+            {
+                "medium": medium,
+                "address": address
+            },
+            ["guest_access_token"], True, 'get_3pid_guest_access_token'
+        )
+        if ret:
+            defer.returnValue(ret["guest_access_token"])
+        defer.returnValue(None)
+
+    @defer.inlineCallbacks
+    def save_or_get_3pid_guest_access_token(
+            self, medium, address, access_token, inviter_user_id
+    ):
+        """
+        Gets the 3pid's guest access token if exists, else saves access_token.
+
+        :param medium (str): Medium of the 3pid. Must be "email".
+        :param address (str): 3pid address.
+        :param access_token (str): The access token to persist if none is
+            already persisted.
+        :param inviter_user_id (str): User ID of the inviter.
+        :return (deferred str): Whichever access token is persisted at the end
+            of this function call.
+        """
+        def insert(txn):
+            txn.execute(
+                "INSERT INTO threepid_guest_access_tokens "
+                "(medium, address, guest_access_token, first_inviter) "
+                "VALUES (?, ?, ?, ?)",
+                (medium, address, access_token, inviter_user_id)
+            )
+
+        try:
+            yield self.runInteraction("save_3pid_guest_access_token", insert)
+            defer.returnValue(access_token)
+        except self.database_engine.module.IntegrityError:
+            ret = yield self.get_3pid_guest_access_token(medium, address)
+            defer.returnValue(ret)