Create availabilities feature

Create availabilities controller
Add date helper to listing controller
Add date helper to reservation controller
Create date helpers
Add availablities method to listing model
Create availiabilites new form
Create availiable date partial
Create availiabilities routes
Add date helpers to reservations show
Add availaibilities to reservation new
While it shouldn't be backed by ActiveRecord, you might consider creating an Availability model (mixing in some ActiveModel stuff). So that the urls are a bit cleaner, you could also mix in validations to get rid of `make_available_on`