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/book.html

@@ -48,7 +48,7 @@
 
     <div class="columns">
         <div class="column is-one-fifth">
-            {% include 'snippets/book_cover.html' with book=book cover_class='is-h-medium-mobile' %}
+            {% include 'snippets/book_cover.html' with book=book cover_class='is-h-m-mobile' %}
             {% include 'snippets/rate_action.html' with user=request.user book=book %}
 
             <div class="mb-3">