add Promo object
`bundle exec annotate`
add join table for promos and access levels
thank you rails for making this easy
add some urls to the page
add promos controller
add ticket_sold
implement promo backend
add bulk generate utility
Merge pull request #467 from ZeusWPI/feature/promo-codes

Promo Codes
Merge branch 'master' into enhanced-payment-flow

Conflicts:
	app/models/event.rb
	app/views/events/_tabs.html.erb
	db/schema.rb
	test/models/included_zone_test.rb
	test/models/period_test.rb
	test/models/registration_test.rb
	test/models/zone_test.rb