Fix display issue with datepicker persisting on page load

Fixes #568

@lukecarbis Please review
This line exists so that the datepicker automatically shows when someone changes the time filter to 'custom' (which saves the user clicking the 'date from' box).

I don't think it's a huge loss - but just wanted to make sure you were aware of this before merging. The alternative would be to write some logic to detect if it's happening on page load or not.