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
Revert "Fix #2971: add reminder events to the bug history"

This reverts commit 3d34fba1c1ecfa703134d69f11a44de1ec9f4e7e following
discussion on the mailing list [1].

[1] http://thread.gmane.org/gmane.comp.bug-tracking.mantis.devel/4330
Create reminder bugnote even if no message text provided

When adding a reminder, a bugnote was actually NOT created when the
reminder text is empty, even if $g_store_reminders = ON.  This was due
to bugnote_add() returning false when $p_bugnote_text is blank as the
code did not take REMINDER type notes into consideration.

Fixes #15744