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/snippets/search_result_text.html

@@ -1,6 +1,8 @@
 {% load i18n %}
 <div class="columns is-mobile is-gapless">
-    {% include 'snippets/book_cover.html' with book=result cover_class='column' img_path=false %}
+    <div class="column is-cover">
+        {% include 'snippets/book_cover.html' with book=result cover_class='is-w-xs is-h-xs' img_path=false %}
+    </div>
 
     <div class="column is-10 ml-3">
         <p>