Improve date and warnings behavior on case activities

Since the due date was removed, the current behavior doesn't always make sense anymore.

1) "Actual Date" label should be "Date and Time" (same as core activity form)

2) Date should default to Now when creating a new activity. For clarity, for editing prepopulated ones it's not an issue anymore since the date that comes up is now the scheduled date.

3) The warnings should work like so:
If Date <= Now AND status = Scheduled
Are you sure? This is a SCHEDULED activity with the ACTUAL DATE in the PAST. Click Cancel to change the date / status. Otherwise, click OK to save.

If Date > Now AND status = Completed
Are you sure? This is a COMPLETED activity with the ACTUAL DATE in the FUTURE. Click Cancel to change the date / status. Otherwise, click OK to save.