Adding usernames to a messaging app may seem like a standard feature, but for Signal, such identifiers were anathema to its mission of total privacy and security — until now. The upcoming 7.0 version adds usernames, but the company’s president, Meredith Whittaker, explained that this was nowhere near as simple a decision as it may sound. The new feature sounds simple: You register a username and that appears instead of your phone number. But why do this at all when everyone already has contact n...