Allow more than 1 student to be ignored on upload

Fixed an error where upon uploading a file to the grade book, you could
only select one student to ignore.  The option would be unavailable for
selection after choosing it for one individual.  A line in JavaScript
assumed that test for > 0 would result in false for a string, which is
incorrect since the string “ignore” is compared lexicographically and
would be greater than zero.  An additional check is necessary when
other values are changed so that it isn’t disabled when we change away
from it.
Test Plan: Upload a grade book file that includes at least 2 unmatched
students (no sis_id or other identifier besides name).  On the
resolution screen, you should be able to choose to ignore more than on
student.  The other entries should still correctly grey out once
selected and become available is selected away from.