Change stats announcement date change rules

A change note should only be provided if changing an already "confirmed"
release date. Otherwise, we ignore the change note.
Only show "change note" text area when required

If the current release date isn't confirmed, then the user won't need
to provide a change note.
Simplify logic on date changes

The logic was way more complicated than it needed to be; if the change
is to a confirmed date, then the change note is required. There is no
need to check exactly what sort of change it is.
Merge pull request #1695 from alphagov/stats-change-note-tweak

Change stats announcement date change rules