Add two new operators to properly handle NULL start or end values

Problem/Motivation

We want to be able to use the opIncludes operator with potentially NULL start or end values.

NULL start or end values are possible when you use the Optional End Date module.

Steps to reproduce

The BETWEEN operator used in the query will exclude all results with NULL start or end values.

Proposed resolution

Replace BETWEEN SQL operator with this simple WHERE condition:

select *
  from m_table
 where ( data >= :start_date
         or :start_date is null
          )
   and ( date <= :end_date
         or :end_date is null
          )

The second operator duplicates the first but generates an index-friendly SQL query by avoiding the DATE_FORMAT function for better performance.

User interface changes

The two new operators added.