[bug #827649] l10n email templates for users and questions

* add the .ltxt domain so email template strings get extracted.

  this only affects jinja templates. the ones that are still
  django templates fail silently during extraction and never make
  it to the .pot file which is fine.

* convert users app and questions app emails from Django to Jinja
  with the exception of apps/users/templates/users/emails/pw_reset.ltxt
  because that template is rendered by Django's PasswordResetForm
  and thus it has to be a Django template until we change Kitsune
  so that jingo overrides the Django templating stuff.

* add required unsubscribe_text macro.

* fix question and answer model makers to create a user with a
  profile.

* create new sumo.email_utils library which simplifies dealing with
  email templates.

* fix all the places in the users and questions apps where we were
  rendering email templates using Django things to use Jingo things
  and use the correct locale.