Allow configuration values to be rearranged in config_defaults

Within config_defaults_inc.php, the following syntax doesn't allow for
configuration values to be rearranged within the file:
$g_some_value = $g_something_else

Instead we should use the following syntax where references to other
values (anywhere in the file, even if not declared yet) are resolved
after the entire config_defaults_inc.php file is loaded:
$g_some_value = '%something_else%'

config_defaults_inc.php

@@ -2471,7 +2471,7 @@
  * $g_delete_bug_threshold.
  * @global string $g_delete_bugnote_threshold
  */
-$g_delete_bugnote_threshold = $g_delete_bug_threshold;
+$g_delete_bugnote_threshold = '%delete_bug_threshold%';
 
 /**
  * Move bug threshold
@@ -2591,23 +2591,23 @@
  * $g_update_bugnote_threshold.
  * @global int $g_bugnote_user_edit_threshold
  */
-$g_bugnote_user_edit_threshold = $g_update_bugnote_threshold;
+$g_bugnote_user_edit_threshold = '%update_bugnote_threshold%';
 
 /**
  * Threshold at which a user can delete his/her own bugnotes.
  * The default value is equal to the configuration setting
  * $g_delete_bugnote_threshold.
  * @global int $g_bugnote_user_delete_threshold
  */
-$g_bugnote_user_delete_threshold = $g_delete_bugnote_threshold;
+$g_bugnote_user_delete_threshold = '%delete_bugnote_threshold%';
 
 /**
  * Threshold at which a user can change the view state of his/her own bugnotes.
  * The default value is equal to the configuration setting
  * $g_change_view_status_threshold.
  * @global int $g_bugnote_user_change_view_state_threshold
  */
-$g_bugnote_user_change_view_state_threshold = $g_change_view_status_threshold;
+$g_bugnote_user_change_view_state_threshold = '%change_view_status_threshold%';
 
 /**
  * Allow a bug to have no category