Change username to user id in SQL schema for alarms

[joelweinberger]

Currently, we are associating alarms with usernames rather than ids. This is pretty inflexible, as it means usernames can never be changed and whatnot, so we really ought to change it to ids instead. Probably not worth doing until we have a larger DB set of changes to do.

[jortizcs]

To be fair, usernames are typical both unique and long-lived. That said, if you think folks may want to change their username, then decoupling the username from a unique identifier is fine.

[romain-fontugne]

This is done in the new db schema. The user is always referred by its _id