Add "Is Between" operator for Date Searches

Some date fields can be searched by date range, while others can be searched by comparing to a single date.  Date range searches can't search based on NULL dates.  Single date searches can't search date ranges.  Combine the two by adding "Is Between" to the drop down list of comparison types for single dates.  When that option is chosen, it means do a date range, so show a second date field for the end date.