Port newsletter preferences to Bedrock

- Create newsletter app and move hacks.mozilla.org.html there
- Set up to display Django messages to the user.
- Make displaying most of the information on the page conditional on having a formset, so we can still use the same template when some error occurs in processing.
- Handle errors by displaying translated messages to the user. Also use Playdoh logging to log errors and debug information.
- If no token given, or token is invalid, let user supply an email
address and sign up for newsletters.