Disable Exam 'Save Answers' button during saving

* Closes #1903
Disable exam 'check answers' button during checks
Re-enable save answers button on success

* Rather than re-enabling the save answers button when the user exits
  the message dialogue, re-enable it on XHR success.
Merge pull request #2118 from Stanford-Online/jrbl-1903-disable-exam-buttons-during-ajax

Disable Exam 'Save Answers' button during saving