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.
# Branch: (no branch)
ugettext_lazy requires unicode argument
Fix formatting on updated page

"Thank you for signing up" was getting formatted differently than the
rest of the thank you message. Put everything in one <div> so it gets
formatted together.

Bug 845904
Remove logic to do anything without a valid token

If the user passes no token to /existing, or the token is not valid,
earlier versions of this code would still allow them to sign up for
new newsletters. That's a change from the current behavior. We've
decided instead of that, we're going to just let them request a link
in the email or unsubscribe all. But that won't happen until a later
bug, so for now, just go back to the current behavior in this case.

Bug 845904