Fix #15678: Add new index to mantis_bug_monitor_table

The new index is on bug_id column, and resolves a performance issue when
filtering on users monitoring issues with any other search criteria when
using match type "Any Condition".
New helper_check_variables_equal() api function

Moved duplicated code in check_checked and check_selected into the new
function.
Add option to perform loose type comparison in check_checked/selected

This is required because in some cases (e.g. when passing an array of
values to compare) it is not feasible or too complex to typecast the
variable.
Fix XML parsing errors

Several occurences were fixed for
 - projects list display with subprojects
 - use date filters