Bug 845904: initial locale when user has none

When the user goes to the /newsletter/existing page but has no
language or country set in ExactTarget, then initialize them on
the form based on the locale from the request URL.
Merge pull request #766 from dpoirier/bug-845904-fix-initial-locale

Bug 845904: initial locale when user has none
Bug 845904 Style updates to existing newsletter pg
More prominent titles for newsletters, better position/style for "English only) text, and better mobile layout
Remove "Our newsletters" heading (Bug 845904)