Make scrolling facets set up their height on init

- Currently scrolling facets adjust their height on open, but some facets will load already open so check this and if they are open already then call setupHeight.
Improved comment

Because 'things' actually meant 'checked checkboxes' and so it should probably have said that.
Merge pull request #15 from alphagov/facet-initialising-fixes

Facet initialising fixes