Don't show "Send code" button when email is already confirmed on Special:ConfirmEmail

Also, consistently use "confirmed", rather than "authenticated", in the
Special:ConfirmEmail and Special:Preferences UI, when messaging whether
or not the user has confirmed an email address.

Bug: 56443
Change-Id: I6fca42ea4568c13bca6b10e19d17e023ed0eacca
Merge "Don't show "Send code" button when email is already confirmed on Special:ConfirmEmail"