Add endpoint that proxies ID server request token and errors if the given email is in use on this Home Server.

synapse/storage/schema/delta/22/user_threepids_unique.sql

@@ -0,0 +1,19 @@
+CREATE TABLE IF NOT EXISTS user_threepids2 (
+    user_id TEXT NOT NULL,
+    medium TEXT NOT NULL,
+    address TEXT NOT NULL,
+    validated_at BIGINT NOT NULL,
+    added_at BIGINT NOT NULL,
+    CONSTRAINT medium_address UNIQUE (medium, address)
+);
+
+INSERT INTO user_threepids2
+	SELECT * FROM user_threepids WHERE added_at IN (
+		SELECT max(added_at) FROM user_threepids GROUP BY medium, address
+	)
+;
+
+DROP TABLE user_threepids;
+ALTER TABLE user_threepids2 RENAME TO user_threepids;
+
+CREATE INDEX user_threepids_user_id ON user_threepids(user_id);