Converting all asserts to ValidationErrors, so that they will be caught and never block actual user behaviors.
guarantee that there's never more than 1 'open' userlog of a single type.  Because if that race condition happens, it's unrecoverable, due to use of get_object_or_None.
making userlog completely robust to simulinaeity
implement multiple users; pre-create them in the view.
setting pct videos by querystring arg
make the number of users configurable as well, defaulting to old value (1)
implementing logouts
fixing js bug
Merge branch 'multiuser_loadtest' of github.com:bcipolli/ka-lite into release-idoc
Merge branch 'robust_userlogs' of github.com:bcipolli/ka-lite into release-idoc

Conflicts:
	kalite/main/models.py
Merge pull request #610 from bcipolli/release-idoc-update1

Release idoc update1
Need to release, so need to merge!