Added a sort to the historical view of snapshots

Sorting by reverse order of snapshot creation date. We don't really
care about records with no snapshot, so the resulting inner join isn't
a problem.

After many attempts to test this, I gave up, because criteria just
doesn't work with unit tests.
Sort historical snapshots for a subject in reverse date order

Passing the sorted collection in the model now, rather than accessing
it from the Subject in the view.
Added a helpful line about the default sort order of historical subject data

Since the dates aren't shown, it's useful to know what seemingly
arbitrary order the records are being sorted in.
Merge pull request #5 from ausaccessfed/feature/#5-sort-history-by-timestamp

Historical Snapshots need to be date and time sorted