Bug 867764 Translate newsletter names and descriptions

* On the /existing page, take the names and descriptions of the newsletters
  that come back from Basket and wrap them for translation before passing them
  to the template.  Some of these are translated already.
* Update the rewrite rule for /newsletter/existing to match all locales.
Merge pull request #1238 from dpoirier/bug-867764-translate-newsletter-names

Bug 867764 Translate newsletter names and descriptions