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/book/edit_book.html

@@ -170,7 +170,7 @@
             <h2 class="title is-4">{% trans "Cover" %}</h2>
             <div class="columns">
                 <div class="column is-narrow">
-                    {% include 'snippets/book_cover.html' with book=book cover_class='is-h-small' %}
+                    {% include 'snippets/book_cover.html' with book=book cover_class='is-h-s' %}
                 </div>
                 <div class="column is-narrow">
                     <div class="block">