cover: Change the logic again:

- Work on feeds.
- Add `.is-cover` to modify the behaviours of columns.
- Only apply logic for dimensions on the cover container; too many contextual side effects otherwise.
- Add classes to dimension and align, including auto margins for flex.
- Rename classes in templates accordingly.

bookwyrm/templates/discover/small-book.html

@@ -6,7 +6,7 @@
     {% with book=book %}
         <a
             href="{{ book.local_path }}"
-        >{% include 'snippets/book_cover.html' with cover_class='is-h-small' %}</a>
+        >{% include 'snippets/book_cover.html' with cover_class='is-h-s' %}</a>
 
         {% include 'snippets/stars.html' with rating=book|rating:request.user %}