Add newsletter description to list on /existing

Bug 857327
Add descriptions to test newsletters

Bug 857327
Bug 857197 Style forms for newsletter pref center
Still needs styles for error/success messages
Merge remote branch 'dpoirier/bug-845904-newsletter-preference-center' into bug-845904-newsletter-preference-center

Conflicts:
	media/css/sandstone/newsletter.less
Indents from 4-spaces to 2-spaces (Bug 845904)
Merge branch 'bug-845904-newsletter-preference-center' into bug-857271-newsletter-languages

Also remove the javascript from existing.html that I was
about to remove anyway.

Conflicts:
	apps/newsletter/templates/newsletter/existing.html
Be more flexible about languages

Changes after our call today:
- Don't care about language when deciding which newsletters to show
- Don't care about language when deciding whether to let them subscribe
  to a newsletter
- Display "English only" on newsletters that are.

Bug 857271