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/feed/feed_layout.html

@@ -37,7 +37,7 @@
                                         aria-label="{{ book.title }}"
                                         aria-selected="{% if active_book == book.id|stringformat:'d' %}true{% elif shelf_counter == 1 and forloop.first %}true{% else %}false{% endif %}"
                                         aria-controls="book-{{ book.id }}">
-                                        {% include 'snippets/book_cover.html' with book=book cover_class='is-h-medium' %}
+                                        {% include 'snippets/book_cover.html' with book=book cover_class='is-h-m' %}
                                     </a>
                                 </li>
                                 {% endfor %}