Set explorer menu media with property instead of class

Fixes #2369

The static() function was being called during app load which caused a crash when the user is using STATICFILES_STORAGE=ManifestStaticFilesStorage, DEBUG=False and haven't yet collected static files.

I've moved it into a property and it's now only called when a view is being rendered. This also is more consistent because we usually set media using properties (and so does Django admin).

CHANGELOG.txt

@@ -19,6 +19,7 @@ Changelog
  * Fix: Streamfields no longer break on validation error
  * Fix: Number of validation errors in each tab in the editor is now correctly reported again
  * Fix: Userbar now opens on devices with both touch and mouse (Josh Barr)
+ * Fix: ``wagtail.wagtailadmin.wagtail_hooks`` no longer calls ``static`` during app load, so you can use ``ManifestStaticFilesStorage`` without calling ``collectstatic`` command
 
 
 1.4.1 (17.03.2016)

docs/releases/1.4.2.rst

@@ -16,4 +16,4 @@ Bug fixes
  * Streamfields no longer break on validation error
  * Number of validation errors in each tab in the editor is now correctly reported again
  * Userbar now opens on devices with both touch and mouse (Josh Barr)
-
+ * ``wagtail.wagtailadmin.wagtail_hooks`` no longer calls ``static`` during app load, so you can use ``ManifestStaticFilesStorage`` without calling ``collectstatic`` command

wagtail/wagtailadmin/wagtail_hooks.py

@@ -1,3 +1,4 @@
+from django import forms
 from django.contrib.auth.models import Permission
 from django.contrib.staticfiles.templatetags.staticfiles import static
 from django.core.urlresolvers import reverse
@@ -11,8 +12,9 @@ from wagtail.wagtailcore.permissions import collection_permission_policy
 
 
 class ExplorerMenuItem(MenuItem):
-    class Media:
-        js = [static('wagtailadmin/js/explorer-menu.js')]
+    @property
+    def media(self):
+        return forms.Media(js=[static('wagtailadmin/js/explorer-menu.js')])
 
 
 @hooks.register('register_admin_menu_item')