Copy shortening and authentication page title simplifications.

* Integrate @ryanfeeley's suggested copy changes.
* Simplify the way the title is displayed on the authentication page. Instead of using JS to display the appropriate title, use CSS.
Clean up the text for transition_to_primary
Fix the failing front end tests

* A .description element no longer exists
* Move the #upgrade_to_primary id to what used to be the .description element.
Merge pull request #3684 from mozilla/issue_3609-dialog_copy

Copy shortening and authentication page title simplifications.

I rebased and can now merge cleanly. Thanks @seanmonstar for reviewing!

merging.