Fix template function getting reused between blocks.

The template function (which is slightly different for each block,
depending on block's configuration) was getting assigned to the global
DragAndDropBlock function. Each block would overwrite the previous
value, which caused bugs.

drag_and_drop_v2/public/js/drag_and_drop.js

@@ -292,8 +292,7 @@ function DragAndDropTemplates(configuration) {
         );
     };
 
-    DragAndDropBlock.renderView = mainTemplate;
-
+    return mainTemplate;
 }
 
 function DragAndDropBlock(runtime, element, configuration) {
@@ -302,7 +301,8 @@ function DragAndDropBlock(runtime, element, configuration) {
     DragAndDropBlock.STANDARD_MODE = 'standard';
     DragAndDropBlock.ASSESSMENT_MODE = 'assessment';
 
-    DragAndDropTemplates(configuration);
+    var renderView = DragAndDropTemplates(configuration);
+
     // Set up a mock for gettext if it isn't available in the client runtime:
     if (!window.gettext) { window.gettext = function gettext_stub(string) { return string; }; }
 
@@ -882,7 +882,7 @@ function DragAndDropBlock(runtime, element, configuration) {
             display_reset_button: Object.keys(state.items).length > 0,
         };
 
-        return DragAndDropBlock.renderView(context);
+        return renderView(context);
     };
 
     /**