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.

Test Plan:
- Upload a grade book file that includes at least 3 unmatched students. You can
  do this by downloading the csv and then deleting their id and sis_id
- On the resolution screen, ignore the first, and match the second
- You should be able to ignore the third student
- Different combinations of ignore and match should work

Change-Id: I922490986b12aedbbae3c46a906ab2b41871f504
Reviewed-on: https://gerrit.instructure.com/37323
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Reviewed-by: Cameron Matheson <cameron@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>