Story 964: Move Echo to stable

Add is-authenticated class to body tag for logged in users
* Hide echo button when not logged in
** Drop the echo log in cta
* Customise Notifications special page for mobile to add a return to link
at top of page
* Move css to stable

Change-Id: Id5ea7a54546fa166fd3ee3d9bb22ca1ab725cffe
Merge "Story 964: Move Echo to stable"