Update email template styling

* Make the header font smaller. It's now 16px instead of 20px.
  I also bumped down the story titles accordingly.
* Added a comma after the story title in the first paragraph.
* Add Google Analytics campaign tracking parameters to all URLs
* Strip HTML tags from summary in plain-text email

Addresses #580
Add HTML email for unpublished notification

Addresses #580
Merge branch 'develop' of github.com:PitonFoundation/atlas into develop
Skip tests that rely on Solr when running under Travis CI