Abstracts ExpirableModel, cleans up references
Adds migrations for changed field
Moves valid_object method to ExpirableMethod
Renames is_open to is_valid
Fixes some onsite check-in bugs
Removes extra unicode method
Moves is_open_qobject to ExpirableModel
Fixes valid_objects method
Merge pull request #714 from learning-unlimited/expirable_model

Adds an abstract class for models that are expirable