remove unnecessary unless statement
add state to lessons table
update lesson state on creation, booking, editing and cancelation
when lesson is canceled, do not allow editing
turn mailer off in development... for now
instructor must confirm or drop lesson after updated by requester
change student to requester across entire app
change 'cancelled' to 'canceled' (proper spelling)
rework logic on lesson show page
additional actions for when an instructor cancels on a lesson

- If no other instructors available, ask student to try a different date/time or cancel the lesson
- If other instructors available, send email to them to take lesson and send email to student letting them know we're looking for another instructor