Remove deleted users from assignments

Signed-off-by: Julius Härtl <jus@bitgrid.net>

lib/Db/AssignedUsersMapper.php

@@ -50,6 +50,12 @@ class AssignedUsersMapper extends DeckMapper implements IPermissionMapper {
 		return $users;
 	}
 
+	public function findByUserId(string $uid): array {
+		$sql = 'SELECT * FROM `*PREFIX*deck_assigned_users` ' .
+			'WHERE `participant` = ?';
+		return $this->findEntities($sql, [$uid]);
+	}
+
 
 	public function isOwner($userId, $cardId) {
 		return $this->cardMapper->isOwner($userId, $cardId);