Bug 901541 - newsletter recovery page

* When someone goes to the /existing page with an invalid token or no
  token, redirect them to a recovery page
* On the recovery page, they can enter their email address and we'll
  send them a message containing a valid link to manage their subscriptions.
* If the email is not known, we tell them to check for typos or go sign up.
Merge pull request #1169 from dpoirier/bug-901541-newsletter-recovery-page

Bug 901541 - newsletter recovery page