Improvements to login form

* Login link now loads the path /:locale/login instead of
  /:locale/session/new.
* If the user ise already logged in when accessing /:locale/login,
  the user will be redirected to /:locale instead of showing the login
  form again.
* ApplicationController now saves the location of each page as a
  return path to the user session. Login uses that session field
  to redirect when the login is successfully completed.