Add a login flow

[gnprice]

As explained in the README, this prototype currently requires baking in a set of account credentials at build time.

By using an appropriate test account, that works fine for development and even for distributing a build among a handful of people. But it'd get unwieldy for distributing a build to more people, especially now (after #9) that the app's functionality isn't all read-only.

So we should add the ability for the user to log in in a normal way, and should then remember the resulting credentials in the app's storage.

[gnprice]

Splitting this issue in two: this one will track just the login flow itself, and I've added #34 for storing the resulting credentials on the device.

To get a reasonable user experience we'll need both. But the login flow is a nontrivial enough UI, and starting to do on-device storage is a large enough infrastructural task, that I think I'll want to merge the one without blocking on the other. So this is the issue we'll close on merging the login flow, and then #34 is an important followup.