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/user/user.html

@@ -36,7 +36,7 @@
             {% for book in shelf.books %}
             <div class="control">
                 <a href="{{ book.local_path }}">
-                {% 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>
             </div>
             {% endfor %}