Bug 823987 - Add default confirmation message for english

If a newsletter doesn't have a custom confirmation message configured,
use the default one. Had to add one for English since we never needed
one before, but now we can do double-opt-in for English too.

Also add confirmation message to admin
Merge pull request #67 from dpoirier/bug-823987-define-default-confirmation-message-for-english

Bug 823987 - Add default confirmation message for english