Waitlists for tea time

- Sign up link changes according to user attendance status
- Emails waitlisted users when a flaking from a full tea time
- Shows list of future waitlisted tea times on profile page
Simple integration test for waitlisting
Mock and test waitlist notifications when flaking
Simplify TT controller logic
Rename and test waitlisted?/attending? methods
Remove commented test code
waitlist email edits, change scheduling page when no tea times, add social share on no tea schedule pg
waitlist to wait list, adjust waiting list modal
Use enums in favour of custom scopes where appropriate
Refactor all waitlist mailers to reside in AttendanceMailer
Fix Ankit's changes to Waitlist mailers
Display spots button on waitlisted tea time
Change waitlist button text when waitlisted
Fix erroneous acceptance of merge conflict
Merge branch 'waitlist'