Minor postgresql migration fixes

- we could leave "empty" (i.e. all null) rows in the
  user_permission_futures table
- if something goes screwy and the user_permission_futures_pkey doesn't
  exist (i.e. because we already dropped it) then the update was
  failing; added an `IF EXISTS` to avoid the error.  (This is unlikely
  to happen in production, but doesn't hurt).

Author: jagerman

sogs/migrations/user_perm_futures.py

@@ -68,7 +68,9 @@ def migrate(conn, *, check_only):
 INSERT INTO user_ban_futures (room, "user", at, banned)
     SELECT room, "user", at, banned FROM user_permission_futures WHERE banned is NOT NULL;
 
-ALTER TABLE user_permission_futures DROP CONSTRAINT user_permission_futures_pkey;
+DELETE FROM user_permission_futures WHERE read IS NULL AND write IS NULL AND uploads IS NULL;
+
+ALTER TABLE user_permission_futures DROP CONSTRAINT IF EXISTS user_permission_futures_pkey;
 ALTER TABLE user_permission_futures DROP COLUMN banned;
 """
         )