Expose Khan.initReportLink to interactive API

Summary: Used for the new "Report a problem" link in Bibliotron!

Test Plan: * n/a

Reviewers: oliver

Reviewed By: oliver

Subscribers: emily, jared, #bibliotron

Differential Revision: https://phabricator.khanacademy.org/D27250

interface.js

@@ -301,6 +301,7 @@ _.extend(Exercises, {
         goToNextProblem: triggerNextProblem,
         skipQuestion: handleSkipEvent,
         showHint: onHintButtonClicked,
+        initReportIssueLink: Khan.initReportIssueLink,
     },
     // These functions allow a client to interact imperatively with the
     // exercises machinery, instead of simulating button clicks, listening for