Change UI to follow Persona's guidelines

- Moved Persona login button from header to login and register pages

- Changed login and register pages to follow the Persona UI guidelines,
  including a Persona icon

- Report the logged-in user's email address or null to Persona's
  navigator.id.watch() on every page

- JavaScript redirects to the dashboard after a successful login,
  instead of just reloading the page (this works from either the login
  or the register page)
Update README
Move the login button to a snippet

Removes some code duplication
Update README