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.less

@@ -0,0 +1,6 @@
+.publishNotebookPanelContent {
+    display: flex;
+    flex-direction: column;
+    flex: 1;
+    overflow-y: auto;
+}