Fix bug publish screenshot (#762)

[Preview this branch](https://cosmos-explorer-preview.azurewebsites.net/pull/762?feature.someFeatureFlagYouMightNeed=true)

The main change in this PR fixes the snapshot functionality in the Publish pane-related components. Because the code cell outputs are now rendered in their own iframes for security reasons, a single snapshot of the notebook is no longer possible: each cell output takes its own snapshot and the snapshots are collated on the main notebook snapshot.
- Move the snapshot functionality to notebook components: this removes the reference of the notebook DOM node that we must pass to the Publish pane via explorer.
- Add slice in the state and actions in notebook redux for notebook snapshot requests and result
- Add post robot message to take snapshots and receive results
- Add logic in `NotebookRenderer` to wait for all output snapshots done before taking the main one collating.
- Use `zustand` to share snapshot between Redux world and React world. This solves the issue of keeping the `PanelContainer` component generic, while being able to update its children (`PublishPanel` component) with the new snapshot.

Additional changes:
- Add `local()` in `@font-face` to check if font is already installed before downloading the font (must be done for Safari, but not Edge/Chrome)
- Add "Export output to image" menu item in notebook cell, since each cell output can take its own snapshot (which can be downloaded)
![image](https://user-images.githubusercontent.com/21954022/117454706-b5f16600-af46-11eb-8535-6bf99f3d9170.png)

src/Explorer/Notebook/NotebookUtil.test.ts

@@ -1,15 +1,15 @@
-import { NotebookUtil } from "./NotebookUtil";
-import * as GitHubUtils from "../../Utils/GitHubUtils";
 import {
-  ImmutableNotebook,
-  MediaBundle,
   CodeCellParams,
-  MarkdownCellParams,
+  ImmutableNotebook,
   makeCodeCell,
   makeMarkdownCell,
   makeNotebookRecord,
+  MarkdownCellParams,
+  MediaBundle,
 } from "@nteract/commutable";
 import { List, Map } from "immutable";
+import * as GitHubUtils from "../../Utils/GitHubUtils";
+import { NotebookUtil } from "./NotebookUtil";
 
 const fileName = "file";
 const notebookName = "file.ipynb";
@@ -131,7 +131,7 @@ describe("NotebookUtil", () => {
   describe("findFirstCodeCellWithDisplay", () => {
     it("works for Notebook file", () => {
       const notebookObject = notebookRecord as ImmutableNotebook;
-      expect(NotebookUtil.findFirstCodeCellWithDisplay(notebookObject)).toEqual(1);
+      expect(NotebookUtil.findCodeCellWithDisplay(notebookObject)[0]).toEqual("1");
     });
   });
 });