Change "Nothing selected" to "No classes" in studentreg

"Nothing selected" is confusing if the user has previously selected lottery
preferences, but didn't get anything -- users empirically think their
preferences were lost.  "No classes" should be accurate for lottery
registration and FCFS registration both with and without a lottery.  Alternate
wording suggestions welcome, but it would be really nice to fix this.
Merge pull request #1477 from learning-unlimited/nothing-selected

Change "Nothing selected" to "No classes" in studentreg