Bug 919042 - /newsletter/existing l10n

* Wrap "Submit" button on updated page for translation.
* Add comment to "existing" page template reminding that the newsletter
  description is already translated in the view code, and doesn't need
  to be wrapped in the template.
Merge pull request #1282 from dpoirier/bug-919042-leftover-l10n

Bug 919042 - /newsletter/existing l10n