Start adding pages to signup for individual newsletters

- Modify the email_newsletter_form() macro so we can use it outside a
  page footer
- Write a new view for single newsletter signup
- Create a template that each of the signup pages can extend, that does
  most of the work so signup pages just have to provide some copy.
Branch: (no branch)
Bug 845904: Continue work on single newsletter signup pages

- Move NewsletterMiddleware from mozorg to newsletter app
- Document how subscription forms get into footers and get processed
- Re-use the newsletter subscription macro
- Move all the doc that was in README.md into Sphinx.

Branch: bug-845904-newsletter-preference-center