CRM-20932 - CRM_Utils_System_Joomla - Fix warning. Define JDEBUG

Background
==========

When running `cv en <extname>` on Joomla, there are repeated warnings, "Use
of undefined constant JDEBUG", and each includes a backtrace.  This is hard
to read.

The bug appears to affect backend scripts; it's discussed with a solution in:

joomla/joomla-cms#11512 (comment)

Before
======

`CRM_Utils_System_Joomla::loadBootStrap()` loads various config files.
However, these may or may not set `JDEBUG`.

After
=====

`CRM_Utils_System_Joomla::loadBootStrap()` defensively sets `JDEBUG` if no
other part of the bootstrap has done so.

CRM/Utils/System/Joomla.php

@@ -559,6 +559,10 @@ public function loadBootStrap($params = array(), $loadUser = TRUE, $throwError =
 
     jimport('joomla.application.cli');
 
+    if (!defined('JDEBUG')) {
+      define('JDEBUG', FALSE);
+    }
+
     // CRM-14281 Joomla wasn't available during bootstrap, so hook_civicrm_config never executes.
     $config = CRM_Core_Config::singleton();
     CRM_Utils_Hook::config($config);