Remove use of 'on' and 'off' strings for storing
boolean filter states

The "sticky issue" and "use date filter" filters are boolean options
that are transmitted via use of 'on' and 'off' strings in the URL.
gpc_get_bool will take these strings and return either true or false (as
a PHP bool type). We were using 'on' and 'off' within filter_api (and
elsewhere) to internally store the state of these two boolean filter
properties.

This has now been replaced with the use of a more sane true/false
representation of the state using a PHP bool type. As a consequence,
checkboxes in the filter UI throughout MantisBT now correctly reflect
the state of the filters as either on/off.