First few changes for the image diffs feature.

data/database.py

@@ -150,10 +150,19 @@ class RepositoryTag(BaseModel):
     )
 
 
+class QueueItem(BaseMode):
+  queue_name = CharField(index=True)
+  body = TextField()
+  available_after = DateTimeField(default=datetime.now, index=True)
+  available = BooleanField(default=True, index=True)
+  processing_expires = DateTimeField(null=True, index=True)
+
+
 def initialize_db():
   create_model_tables([User, Repository, Image, AccessToken, Role,
                        RepositoryPermission, Visibility, RepositoryTag,
-                       EmailConfirmation, FederatedLogin, LoginService])
+                       EmailConfirmation, FederatedLogin, LoginService,
+                       QueueItem])
   Role.create(name='admin')
   Role.create(name='write')
   Role.create(name='read')