Merge pull request #1072 from dpoirier/bug-893144-newsletter-existing-styling

Bug 893144 - /newsletter/existing/ styling
Fix bug 893187 - Add Optimizely code snippet to mozilla.org homepage
Merge pull request #1075 from craigcook/bug-893187-optimizely-home-page
Bug 892696 - Change default language for /newsletter

On /newsletter and footer email forms, if the user's locale is not
for a language that our newsletters support, then instead of
defaulting the language field to 'en', default to having it not
set, to force the user to choose one of the supported languages
when subscribing instead of just clicking through and maybe not
wanting English but getting it anyway.