OF-1147: Ensure Admin plugin is fully loaded.

Every plugin that adds to the Admin panel has the admin plugin as
an implicit dependency. Make sure that the admin plugin is loaded,
before any of the other plugins are loaded.

src/java/org/jivesoftware/openfire/container/PluginMonitor.java

@@ -277,6 +277,13 @@ public class PluginMonitor
                             } );
                         }
 
+                        // Before running any plugin, make sure that the admin plugin is loaded. It is a dependency
+                        // of all plugins that attempt to modify the admin panel.
+                        if ( pluginManager.getPlugin( "admin" ) == null )
+                        {
+                            pluginManager.loadPlugin( dirs.getFirst().get( 0 ) );
+                        }
+
                         // Hierarchies could be processed in parallel. This is likely to be beneficial during the first
                         // execution of this monitor, as during later executions, most plugins will likely already be loaded.
                         final int parallelProcessMax = JiveGlobals.getIntProperty( "plugins.loading.max-parallel", 4 );