Don't allow invalid date ranges in booking methods

How to reproduce

• Problem 1
  • Create a course
  • Add a booking method free, code or paypal
  • In each of the methods define an end date that is before the start date 
  • => NOK: no error appears, system accepts invalid date range
• Problem 2
  • In the PayPal method, enter 0.123 as price
  • => NOK: no error appears, even though only 2 digits are supported in the back end

Expected behaviour

• Problem 1
  • When the end date is before the start date, an error appears and the form can not besaved
  • Start date and end date using the same date is allowed. Meaning is that the booking method is only valid at this date
• Problem 2
  • When entering a number with more than 2 digits an error message is displayed and the form can not be saved