validates_date requires gem & additional date validation

[dwiehoff]

• validates_date only works with the respective gem installed: https://stackoverflow.com/a/13264276
• validating that end_date comes after start_date: https://medium.com/lightthefuse/ruby-on-rails-date-validation-in-a-booking-and-disabling-dates-in-date-picker-3e5b4e9b4640