Fixed #3078 -- Add empty panel attributes

Accessing any of these attributes before importing `wagtailadmin.edit_handlers` would raise a `AttributeError`. In some cases this would mask the actual error; e.g. forgetting to import `wagtailadmin.edit_handlers`.

CONTRIBUTORS.rst

@@ -174,6 +174,7 @@ Contributors
 * Axel Haustant
 * Henk-Jan van Hasselaar
 * alexfromvl
+* Jaap Roes
 
 Translators
 ===========

wagtail/wagtailcore/models.py

@@ -396,6 +396,12 @@ class Page(six.with_metaclass(PageBase, AbstractPage, index.Indexed, Clusterable
     # Do not allow plain Page instances to be created through the Wagtail admin
     is_creatable = False
 
+    # Define these attributes early to avoid masking errors. (Issue #3078)
+    # The canonical definition is in wagtailadmin.edit_handlers.
+    content_panels = []
+    promote_panels = []
+    settings_panels = []
+
     def __init__(self, *args, **kwargs):
         super(Page, self).__init__(*args, **kwargs)
         if not self.id and not self.content_type_id: