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/import_status.html

@@ -125,7 +125,7 @@
             <td>
                 {% if item.book %}
                 <a href="/book/{{ item.book.id }}">
-                {% include 'snippets/book_cover.html' with book=item.book cover_class='is-h-small' %}
+                {% include 'snippets/book_cover.html' with book=item.book cover_class='is-h-s' %}
                 </a>
                 {% endif %}
             </td>