WIP: manage and delete account screens.

Start on the manage page and delete account screen.

* Add two new screens, /manage and /delete_account
* Add new auth-server functions to the fxa-client
* Allow the fxa-client to handle updating the Session state
* When visiting /, redirect to /signup
* Remove the old /settings page
* Remove a lot of View display boilerplate by creating a generic function showView

issue #254