extensionSystem: Allow disabling extensions when globally disabled

The canChange property was not actually reflecting whether
the enableExtension or disableExtension can change the underlying
setting. Instead the property was showing whether such a change will
have an effect.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/695

js/ui/extensionSystem.js

@@ -367,10 +367,7 @@ var ExtensionManager = class {
             ? DISABLE_USER_EXTENSIONS_KEY
             : ENABLED_EXTENSIONS_KEY;
 
-        extension.canChange =
-            !hasError &&
-            global.settings.is_writable(changeKey) &&
-            (isMode || !modeOnly);
+        extension.canChange = global.settings.is_writable(ENABLED_EXTENSIONS_KEY);
     }
 
     _getEnabledExtensions() {