Update math4.css

Addressed bug 2647
Fixed broken disabled buttons
Conditional release initial commit. Allows instructor to specify that a set will not become visible until a sufficiently high score is obtained on another set.
Updates to conditional release. Removed relax_restricted_release. Added feature to warn and ignore nonexistent set names in the restricted release list. Now gives message about restricted release instead of toggling visibility.
Cleanup from rebase
Some conditonal release fixes and tweaks.
Merge pull request #130 from aubreyja/conditional_release

Conditional release