fixing bug where deleted video files did not invalidate their cache.  Also stop auto-generating cache files by default, as it's very slow and videoscan is called as a function for update page loading.

Conflicts:
	kalite/main/management/commands/videoscan.py
duplicating change in another branch

Conflicts:
	kalite/securesync/api_views.py
	kalite/utils/decorators.py
fixing two caching bugs, based on deletion.  Adding test logic for setting up temporary cache and content_root, so test scripts of caching and video manipulation can occur without touching any existing cache items or videos.

Conflicts:
	kalite/main/tests/__init__.py
set _web_cache on demand, to avoid errors when caching is disabled.
Merge branch 'release-0.10' of github.com:learningequality/ka-lite into videoscan_caching_bug

Conflicts:
	kalite/securesync/api_views.py
Completing tests, code review comments.
removing unnecessary skipIf decorators, adding better comments to tests.
Merge pull request #423 from bcipolli/videoscan_caching_bug

Fixing failure to invalidate cache when videos are deleted. (final comment was incorporated before merge!)