Fixes incorrect flash message on confirmation

The `:confirmed` default message in `devise.en.yml` used to say "You are now signed in." It is no longer the default behavior in v3.1.0 to automatically sign in a confirmed user. The new default is to confirm the account and redirect to the new session page without signing in the user (unless `Devise.allow_insecure_sign_in_after_confirmation` is set to true).

The default message shouldn't lie :wink:

This pull request renames the old message to `:confirmed_and_signed_in` and changes the :confirmed message to be appropriate for the new default behavior. The new `:confirmed` message reads:

> "Your account was successfully confirmed. Please sign in."