add fixup_1.sql, which corrects the UNIQUE error on user_contacts

- The user_contacts table contained an incorrect UNIQUE constraint
  on wa_jid that meant only one user of the bridge could have any
  given whatsapp user in their contacts.
- This is fixed in fixup_1.sql, which should be applied over schema.sql.

fixup_1.sql

@@ -0,0 +1,18 @@
+BEGIN;
+CREATE TABLE new_user_contacts (
+       id INTEGER PRIMARY KEY,
+       user_id INT NOT NULL REFERENCES users,
+       wa_jid VARCHAR NOT NULL,
+       subscription_state VARCHAR NOT NULL DEFAULT 'none',
+       avatar_url VARCHAR,
+       name VARCHAR,
+       notify VARCHAR,
+       status VARCHAR,
+       UNIQUE(user_id, wa_jid)
+);
+
+INSERT INTO new_user_contacts SELECT * FROM user_contacts;
+DROP TABLE user_contacts;
+ALTER TABLE new_user_contacts RENAME TO user_contacts;
+CREATE INDEX user_contacts_idx ON user_contacts (user_id, wa_jid);
+COMMIT;