Related to MDL-83716, but when creating events in the calendar.

Replication:

1. In sandbox.moodledemo.net, go to a course and add the calendar block
2. Go to the course calendar
3. Create a new event
4. In the new event dialog, click 'show more'

Observed: Even though the 'until' option is not selected, you can still change dates using the calendar icon

Expected: When the option is not selected, you shouldn't be capable of changing that date by any means. This could be confusing for end users.