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/__snapshots__/PublishNotebookPaneComponent.test.tsx.snap

@@ -56,6 +56,14 @@ exports[`PublishNotebookPaneComponent renders 1`] = `
               "key": "Custom Image",
               "text": "Custom Image",
             },
+            Object {
+              "key": "Take Screenshot",
+              "text": "Take Screenshot",
+            },
+            Object {
+              "key": "Use First Display Output",
+              "text": "Use First Display Output",
+            },
           ]
         }
       />