Added support for taking screenshot during Notebook publish to Gallery (#108)

* Added support for taking screenshot

- Screenshot is taken using html2canvas package
- Converted to base 64 and uploaded to metadata
- For Using first display output
  - Notebok object is passed instead of string, to publish pane
  - The first cell with output present is parsed out
  - The dom is also parsed to get corresponding div element to take screenshot of the first output

* fixed bug

* Addressed PR comments

- FIxed bug that didn't capture screenshot when mutiple notebook tabs are opened

* removed unnecessary dependencies

* fixed compile issues

* more edits

src/Explorer/Notebook/NotebookComponent/reducers.ts

@@ -82,6 +82,19 @@ export const cdbReducer = (state: CdbRecord, action: Action) => {
       });
       return state;
     }
+
+    case cdbActions.UPDATE_NOTEBOOK_PARENT_DOM_ELTS: {
+      const typedAction = action as cdbActions.UpdateNotebookParentDomEltAction;
+      var parentEltsMap = state.get("currentNotebookParentElements");
+      const contentRef = typedAction.payload.contentRef;
+      const parentElt = typedAction.payload.parentElt;
+      if (parentElt) {
+        parentEltsMap.set(contentRef, parentElt);
+      } else {
+        parentEltsMap.delete(contentRef);
+      }
+      return state.set("currentNotebookParentElements", parentEltsMap);
+    }
   }
   return state;
 };