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/NotebookComponentBootstrapper.tsx

@@ -17,7 +17,7 @@ import {
 } from "@nteract/core";
 import * as Immutable from "immutable";
 import { Provider } from "react-redux";
-import { CellType, CellId, toJS } from "@nteract/commutable";
+import { CellType, CellId, ImmutableNotebook } from "@nteract/commutable";
 import { Store, AnyAction } from "redux";
 
 import "./NotebookComponent.less";
@@ -71,14 +71,14 @@ export class NotebookComponentBootstrapper {
     );
   }
 
-  public getContent(): { name: string; content: string } {
+  public getContent(): { name: string; content: string | ImmutableNotebook } {
     const record = this.getStore()
       .getState()
       .core.entities.contents.byRef.get(this.contentRef);
-    let content: string;
+    let content: string | ImmutableNotebook;
     switch (record.model.type) {
       case "notebook":
-        content = JSON.stringify(toJS(record.model.notebook));
+        content = record.model.notebook;
         break;
       case "file":
         content = record.model.text;