Fixes incorrect flash message on confirmation

The :confirmed default message in devise.en.yml
used to say "You are now signed in." This is no
longer the default behavior in v3.1.0.

This commit renames that message to
:confirmed_and_signed_in and changes the :confirmed
message to be appropriate for the default post-
confirmation location (which is now the new session
page). The new :confirmed message reads:

"Your account was successfully confirmed. Please
sign in."
Merge pull request #2604 from gregates/revise_locale_messages

Fixes incorrect flash message on confirmation