Convert NewsletterResource to DRF (bug 910564)

https://bugzilla.mozilla.org/show_bug.cgi?id=910564

Questions for @washort who wrote the initial implementation:
- Is this supposed to work for anonymous users ? (I forced the user to be authenticated and added a test, but wasn't sure, it's not clear from the documentation)
- For authenticated users, are they supposed to be able to give a different email than the one attached to their UserProfile ? (I left it as is but maybe this is something we need to fix ?)