student can cancel lesson from link on lesson page
instructor can cancel on a lesson
add additional lesson details to lesson page and emails
only show edit and cancel links if current user is lesson's student
send cancellation email to instructor when student cancels lesson
instructor can cancel on a lesson
clean up Lessons Controller by moving logic to private methods
remove RAILS_ENV flag for mailer and add back to development
remove line from new lesson request email (student_count does not exist yet)
send email to instructor when lesson is updated