This will be used for the date range filters for BigCZ catalog search.

Adds UI elements for collecting start and end date filters and passes
them along to the existing API endpoint using the pre-exisitng query
string parameters.  Does some light validation to ensure that
before < after, which would otherwise raise exceptions on the backend.

Uses formatted numbers and non-breaking spaces between the value and
units.

The h2 element with the Data Source title on the search page had its
size restricted by these css rules. Removing them allows the full
element to be shown, as in other sidebar pages.

When a date filter is given a value, a clear icon is added to the
element for easy removal.