Mexico city — Mexico is developing a cellphone app that will allow migrants to warn relatives and local consulates if they think they are about to be detained by U.S. immigration authorities, a senior official said Friday. The move comes in response to President-elect Donald Trump’s threats to carry out mass deportations after he takes office on Jan. 20. The app has been rolled out for small-scale testing and “appears to be working very well,” said Juan Ramon de la Fuente, Mexico’s secretary of foreign affairs. He said the app would allow users to press a tab that would send