Prevent cooldown horse from being registered

[parkerdavis1]

When creating a new event, a list of horses that have cooldown conflicts during the startdate are displayed in a toast.

Ideally there would be a red border around the listbox on an error but that can wait until later IMO.

Still need to add cooldown checking to the Edit Event Details dialog.

[parkerdavis1]

Added cooldown logic for editing events.