Assume english format when parsing dates on stats announcement filter

Bug affected the stats announcement filter (https://www.gov.uk/government/statistics/announcements (yes, it's supposed to be empty, the feature's not in use yet and isn't discoverable)).

Previously, the date fields would assume an American date format when ambiguous, 
so entering "12/06/2010" would parse out as 6th December 2010.

I've taken a look to see if there's any way to configure Chronic site-wide, but that's a no-go.

Story: https://www.pivotaltracker.com/story/show/73205986