Added file cache.py

That file should have been added in 443f3c6b5f95425955328fd7d59296cbcb329ba4
Created a new template tag to detect if a course has available assets

A template tag to detect if a course has available assets has been created.
File assets/utils.py was created with helper functions course_has_assets and
course_get_assets.

Base course template has been modified to include a link to the assets view.