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/NotebookUtil.ts

@@ -1,5 +1,5 @@
 import path from "path";
-import { ImmutableNotebook } from "@nteract/commutable";
+import { ImmutableNotebook, ImmutableCodeCell, ImmutableOutput } from "@nteract/commutable";
 import { NotebookContentItem, NotebookContentItemType } from "./NotebookContentItem";
 import { StringUtils } from "../../Utils/StringUtils";
 import * as GitHubUtils from "../../Utils/GitHubUtils";
@@ -100,4 +100,30 @@ export class NotebookUtil {
     const basePath = path.split(contentName).shift();
     return `${basePath}${newName}`;
   }
+
+  public static findFirstCodeCellWithDisplay(notebookObject: ImmutableNotebook): number {
+    let codeCellCount = -1;
+    for (let i = 0; i < notebookObject.cellOrder.size; i++) {
+      const cellId = notebookObject.cellOrder.get(i);
+      if (cellId) {
+        const cell = notebookObject.cellMap.get(cellId);
+        if (cell && cell.cell_type === "code") {
+          codeCellCount++;
+          const codeCell = cell as ImmutableCodeCell;
+          if (codeCell.outputs) {
+            const displayOutput = codeCell.outputs.find((output: ImmutableOutput) => {
+              if (output.output_type === "display_data" || output.output_type === "execute_result") {
+                return true;
+              }
+              return false;
+            });
+            if (displayOutput) {
+              return codeCellCount;
+            }
+          }
+        }
+      }
+    }
+    throw new Error("Output does not exist for any of the cells.");
+  }
 }