Bug 901541 - newsletter recovery page

Ready for review and merge

* When someone goes to the /existing page with an invalid 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.

Right now, this doesn't do any validation on the front-end, other than email address syntax checking. It's up to basket to decide what to do with an email address that isn't currently subscribed.

Prereqs:

* https://github.com/mozilla/basket-client/pull/16
* https://github.com/mozilla/basket/pull/75