[UX] A user with a valid schedule can check invalid sections

[jeffh]

Fall 2014:
"To reproduce the bug, jump into yacs.me, go to CSCI, select Section 8
of CSCI-1100, then go back to the catalog and select Section 12 of
CHEM-1100 -- all of the sections of CHEM-1100 suddenly are available
(even though many of them conflict with CSCI-1100)."