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".

admin/schema.php

@@ -612,3 +612,4 @@ function installer_db_now() {
 $upgrade[] = Array( 'AddColumnSQL', Array( db_get_table( 'custom_field_string' ), "
 	text		XL  			NULL DEFAULT NULL " ) );
 $upgrade[] = Array( 'UpdateFunction', 'update_history_long_custom_fields' );
+$upgrade[] = Array( 'CreateIndexSQL', Array( 'idx_bug_id', db_get_table( 'bug_monitor' ), 'bug_id' ) );