Create a new cancellation for a program.

Business Rules:

• The program must not be archived.
• The start date must fall within the program's start and end date range.
• The end date (if provided) must not be before start.
• The end date (if provided) must also fall within the program's date range.
• Past cancellations are allowed — start can be a date in the past as long as it is within the program's date range.
• If the program is already published, creating a cancellation will automatically cancel all existing events that fall within the specified date range.