Mentor can only input datetimes that pass validation when adding availability

[esparr]

We need validation for the datetime inputs for a mentor’s availability.

Validation Cases

• start time cannot be before end time
• start time nor end time can be in past
• availability cannot be longer than 12 hours

Acceptance Criteria:

• The Validation Cases should be run every time a user attempts to create an availability.
• If any of these cases are not met, there are 2 consequences:
  - an API call should not be made
  - an error message should be displayed for the user

If this seems like too big a ticket, you can break it up into 3 tickets, one for each Validation Case. The 2 Consequences will be required for each ticket.