Desktop: Fixes: Display fallback icons on a reasonable size on HTML notes

[pedr]

Fixes https://discourse.joplinapp.org/t/huge-black-arrows-in-html-emails/41498

Summary

The issue the user reports happens when an HTML note (webclipper/email-to-note) is synchronized but not all resources have been downloaded yet. When that happens a fallback icon is rendered in the place of it.

The problem is that for the icon to be in a reasonable size, some CSS rules are applied.

The HTML note shouldn't need most of our CSS rules, so these basic rules are only applied to the Markdown notes.

Testing

1. Use webclipper to save a page with a lot of images
2. Synchronize to a remote server
3. Remove local files (e.g.: delete ~/.config/joplindev-desktop/)
4. Reopen the Desktop app
5. When the Fetch Items has finished, the note from the clipper should appear in the list
6. Stop the synchronisation by shutting down the server/shutting down the internet or observe the note while the resource is being downloaded
7. The icons should be shown in a reasonable size like the image below