Introduce Resource.Thumbnail and replace Resource.image with Resource.thumbnail. Implement
Analyzer.thumbnail() and use it for images, webpages and YouTube videos. Show thumbnail in
micro-image, micro-video and micro-link.

Add Expect.dict(), make Application.update() and Server.start() async, include file storage in
ApplicationUpdateTest and remove support for recursive Analyzer.analyze().

Also make Resource a dataclass and type touched code.

Deprecated: Resource.image.

Breaking changes:

* Replace Resource.image with Resource.thumbnail
* Make Application.update() and Server.start() async

Close #76.