Notebooks Gallery (#59)

* Initial commit

* Address PR comments

* Move notebook related stuff to NotebookManager and dynamically load it

* Add New gallery callout and other UI tweaks

* Update test snapshot

src/Explorer/Controls/NotebookViewer/NotebookViewerComponent.less

@@ -1,7 +1,7 @@
 @import "../../../../less/Common/Constants";
 
 .notebookViewerContainer {
-  padding: @DefaultSpace;
+  padding: 30px;
   height: 100%;
   width: 100%;
   overflow-y: auto;