This reverts commit c6192b8b40656c44ba0a89cdd569fb0c0e4f87c4.
The caching does little to save performance and in the case of whole
course interation, it has a netgative performance impact.
With this change, whenever a IntegritySignature is created or retrieved, the result will be cached into mem cache for future use. This will save round trip to database significantly for honor code check
MST-861. The Django admin page for IntegritySignature only displays
the user and course_key for a signature. This adds the created and modified
fields, and will order signatures by the modified field.
