Setting default value for kqList in Asset to null
Created cache for assets

cache module was created in assets application, and functions were defined in
order for it to be able to store if a course has bookable assets.
Models file was modified to invalidate the cache entry of a course each time a
kq is asigned or removed from an asset.
Allow an asset not to be selectable in any KnowledgeQuantum
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.