login: Support logging out of an account

[chrisbobbe]

Screenshots coming soon. :)

Fixes: #463

[chrisbobbe]

[chrisbobbe]

This implements the logic where per-account routes on the nav are removed on logout, but it doesn't animate their exits; they just disappear suddenly. I think that's probably fine, and I didn't find a way to add the animation in.

To test a scenario where there are per-account routes when the logout happens, you can add a button to the "under construction" screen and copy-paste _logOutAccount into its press handler.

[chrisbobbe]

Marked as a draft because in particular I think this may be lacking some test coverage that we'll want; for example, of the various if (maybeAccount == null) return;s added in PerAccountStore. But in that example, at least, I'm not sure if there might be a nicer way to write the app code. I added those early returns to avoid having ApiConnection.send get called after the ApiConnection was closed, which is a goal, as I think we discussed in the office on Friday.