Bug 845904: Port newsletter preferences to Bedrock

[Status: being tested on demo3]

See https://bugzilla.mozilla.org/show_bug.cgi?id=845904

- Create newsletter app and move hacks.mozilla.org.html there
- Set up to display Django messages to the user.
- Allow user with token to update their subscriptions, same as the current page.
- Allow user to update their preferred format, language, and country. (Fixing bugs, these don't work on the current page)
- Handle errors by displaying translated messages to the user. Also use Playdoh logging to log errors and debug information.

- User will not be able to change their email address yet, that's a more complicated question that will be addressed later. See https://bugzilla.mozilla.org/show_bug.cgi?id=742951