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.
Handle zip files with spaces in filenames

The filename extraction code was not coping with zip files containing
files that have spaces in their filenames. This updates the code to
handle such cases.
Merge pull request #1696 from alphagov/better-zip-handling

Handle zip files with spaces in filenames
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.
Minor text amendments

as per Zen ticket #811289
Merge pull request #1697 from alphagov/update_history_page

Updating No 10 history page with minor text amendments
Merge pull request #1695 from alphagov/stats-change-note-tweak

Change stats announcement date change rules