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/Panes/PublishNotebookPaneComponent.test.tsx

@@ -10,6 +10,8 @@ describe("PublishNotebookPaneComponent", () => {
       notebookTags: "tag1, tag2",
       notebookAuthor: "CosmosDB",
       notebookCreatedDate: "2020-07-17T00:00:00Z",
+      notebookObject: undefined,
+      notebookParentDomElement: undefined,
       onChangeDescription: undefined,
       onChangeTags: undefined,
       onChangeImageSrc: undefined,