This commit moves the signin and account control (and dropdown) into its
own directive.

To make it easier to understand what's going on at load, I've also
changed the signalling of user signed-in/signed-out state to an explicit
`status` field on the `auth` object, rather than relying on a tristate
of {undefined, null, user object}. This results in templates that are
marginally more verbose, but substantially clearer in intent.

Lastly, I've moved the computation of the `username` and `provider`
properties into AppController so that it doesn't need a `$scope.$watch`.