show warning on navigate away from import page only for first two stages

TNL-554

cms/static/js/factories/import.js

@@ -28,6 +28,7 @@ define([
             autoUpload: false,
             add: function(e, data) {
                 CourseImport.clearImportDisplay();
+                CourseImport.okayToNavigateAway = false;
                 submitBtn.unbind('click');
                 file = data.files[0];
                 if (file.name.match(/tar\.gz$/)) {
@@ -97,7 +98,9 @@ define([
             },
             start: function(event) {
                 window.onbeforeunload = function() {
-                    return gettext('Your import is in progress; navigating away will abort it.');
+                    if (!CourseImport.okayToNavigateAway) {
+                        return "${_('Your import is in progress; navigating away will abort it.')}";
+                    }
                 };
             },
             sequentialUploads: true,

cms/static/js/views/import.js

@@ -49,6 +49,7 @@ define(
          */
         var getStatus = function (url, timeout, stage) {
             var currentStage = stage || 0;
+            if (currentStage > 1) { CourseImport.okayToNavigateAway = true; }
             if (CourseImport.stopGetStatus) { return ;}
 
             if (currentStage === 4) {
@@ -87,6 +88,10 @@ define(
              * progress.
              */
             stopGetStatus: false,
+            /**
+             * Whether its fine to navigate away while import is in progress
+             */
+            okayToNavigateAway: false,
 
             /**
              * Update DOM to set all stages as not-started (for retrying an upload that