Added support for custom image upload during publish to Gallery (#99)

* Added support for custom image upload

- Dropdown gives an option for URL or image upload
- Preview shows how the card will be displayed in the gallery
- base64 converted image stored in metadata document
- Max limit is 1.5MiB for the image

* fixed lint errors

* addressed PR comments

- Added test

* added snapshot

* fixed failing test

src/Explorer/Panes/PublishNotebookPaneComponent.test.tsx

@@ -0,0 +1,23 @@
+import { shallow } from "enzyme";
+import React from "react";
+import { PublishNotebookPaneComponent, PublishNotebookPaneProps } from "./PublishNotebookPaneComponent";
+
+describe("PublishNotebookPaneComponent", () => {
+  it("renders", () => {
+    const props: PublishNotebookPaneProps = {
+      notebookName: "SampleNotebook.ipynb",
+      notebookDescription: "sample description",
+      notebookTags: "tag1, tag2",
+      notebookAuthor: "CosmosDB",
+      notebookCreatedDate: "2020-07-17T00:00:00Z",
+      onChangeDescription: undefined,
+      onChangeTags: undefined,
+      onChangeImageSrc: undefined,
+      onError: undefined,
+      clearFormError: undefined
+    };
+
+    const wrapper = shallow(<PublishNotebookPaneComponent {...props} />);
+    expect(wrapper).toMatchSnapshot();
+  });
+});