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/NotebookComponentAdapter.tsx

@@ -8,6 +8,7 @@ import { actions, createContentRef, createKernelRef, selectors } from "@nteract/
 import VirtualCommandBarComponent from "./VirtualCommandBarComponent";
 import { NotebookContentItem } from "../NotebookContentItem";
 import { NotebookComponentBootstrapper } from "./NotebookComponentBootstrapper";
+import { CdbAppState } from "./types";
 
 export interface NotebookComponentAdapterOptions {
   contentItem: NotebookContentItem;
@@ -18,6 +19,7 @@ export interface NotebookComponentAdapterOptions {
 
 export class NotebookComponentAdapter extends NotebookComponentBootstrapper implements ReactAdapter {
   private onUpdateKernelInfo: () => void;
+  public getNotebookParentElement: () => HTMLElement;
   public parameters: any;
 
   constructor(options: NotebookComponentAdapterOptions) {
@@ -44,6 +46,11 @@ export class NotebookComponentAdapter extends NotebookComponentBootstrapper impl
         })
       );
     }
+
+    this.getNotebookParentElement = () => {
+      const cdbAppState = this.getStore().getState() as CdbAppState;
+      return cdbAppState.cdb.currentNotebookParentElements.get(this.contentRef);
+    };
   }
 
   protected renderExtraComponent = (): JSX.Element => {