CiviMail - Modify Send Mailing form buttons to clarify functionality on Schedule and Send page

Current buttons on Schedule or Send page are confusing. "Done" button should be relabeled to "Submit Mailing". "Cancel" button and "Save and Continue" are redundant on this page. Remove "Cancel" - and rename other button "Continue Later".

Also added visual separation between the primary action button (Next >> ) and the less used controls (Save and Continue Later, Cancel).