[Offline authentication] Allow switching users while offline

[gino-m]

trafficstars

• [ ] Make all data in local db per-user
• [ ] Allow switching among available accounts in drawer (docs). Fetch and store Firestore auth token on switch.
• [ ] Add link to manage accounts / add account to system

[gino-m]

Changing the flow from "sign in / sign out" to "add/remove user" would reduce change user signs out while offline and ends up not being able to use the app.

[scolsen]

Closing in favor of #769. We won't support multi-user for now.