PermissionsDao: Add clearForPage()

database/permissionsdao.h

@@ -9,6 +9,8 @@ class PermissionsDao
 	PermissionsDao();
 	virtual std::optional<Permissions> find(std::string pagename, std::string username) = 0;
 	virtual void save(std::string pagename, std::string username, Permissions perms) = 0;
+	virtual void clearForPage(std::string pagename) = 0;
+
 	virtual ~PermissionsDao() = default;
 };