Push PerAccountRoot under MaterialApp, into each individual page widget

[gnprice]

This will let us have a single MaterialApp that can navigate between pages that are for different accounts, as well as pages like the login flow and account-switch UI that are for no particular account at all. That makes it a necessity for doing #11 in a reasonable way.

One part of this will be to let the PerAccountRoots for a given account all point to a single PerAccountStore. That's this TODO from #20 which I just sent:

  // TODO add a registry of [PerAccountStore]s, like the latter's of [MessageListView]
  //   That will allow us to have many [PerAccountRoot] widgets for a given
  //   account, e.g. at the top of each page; and to access server data from
  //   outside any [PerAccountRoot], e.g. for handling a notification.

The other part to take care of will be arranging that the API we provide for ourselves to use when we're adding new routes, and navigating to routes, isn't too annoying or error-prone. I have some thoughts and we'll see how it looks after I experiment.