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

package.json

@@ -54,6 +54,7 @@
     "es6-symbol": "3.1.3",
     "eslint-plugin-jest": "23.13.2",
     "hasher": "1.2.0",
+    "html2canvas": "1.0.0-rc.5",
     "immutable": "4.0.0-rc.12",
     "is-ci": "2.0.0",
     "jquery": "3.5.1",