Bug 865058 - fix newsletter ordering

* Fix sorting of newsletters on /existing page so they're sorted by
  the newsletters' `order` field
* Add a test
Merge pull request #1080 from dpoirier/bug-865058-fix-newsletter-ordering

Bug 865058 - fix newsletter ordering
Fix bug 862842 - Add Webmaker Mentor list signup to /contribute

When users sign up on https://mozilla.org/contribute and select "Education"
from the drop down list, an additional checkbox to sign up for the Webmaker
Mentor mailing list should appear.