Make the homepage IclaSignatures#index
Rename all instances of "login" to "sign in"

This commit also adds 301 redirects for /login, /signin, /logout, and
/signout that redirect to the "official" sign in or sign out page. In
ae97cae9, we changed the text that appears to the user, but this commit
changes the underlying methods to also use "sign in" and "sign out", now that
the project is open source and hackable :smile:.
Add internationalization and redirect users who are already signed in
Add specs for testing the controller redirect when signed in