library: initialize the content-type mapping lazily

This is required for Django 1.7 as the contenttypes application is not
initialized properly when the cell data models are loaded and added to
the registry.

combo/data/library.py

@@ -21,17 +21,23 @@ class Library(object):
     providing page cells."""
 
     def __init__(self):
-        self.classes = {}
+        self.classes = []
+        self.classes_by_content_id = None
 
     def get_cell_classes(self):
-        return self.classes.values()
+        return self.classes
 
     def get_cell_class(self, content_type_id):
-        return self.classes[int(content_type_id)]
+        if self.classes_by_content_id is None:
+            # initialize the dictionary
+            self.classes_by_content_id = {}
+            for klass in self.classes:
+                content_type = ContentType.objects.get_for_model(klass)
+                self.classes_by_content_id[content_type.id] = klass
+        return self.classes_by_content_id[int(content_type_id)]
 
     def register_cell_class(self, klass):
-        content_type = ContentType.objects.get_for_model(klass)
-        self.classes[content_type.id] = klass
+        self.classes.append(klass)
         return klass