cover: Handle covers with specific heights:

- Have an explicit contextual class on `cover-container`.
- Use more flexible, consistent and searchable variable name for passing classes to covers.
- Consistently use `'…'` with django variables.
- Give the option to not hide covers to screen readers.
- consitently give a title to the cover container if `alt_text` exists.
- [lists] Remove `.content` which is applying too extensive default styles.

bookwyrm/templates/snippets/status/generated_status.html

@@ -8,7 +8,7 @@
 <div class="columns is-mobile">
     <div class="column is-narrow">
         <div>
-            <a href="{{ book.local_path }}">{% include 'snippets/book_cover.html' with book=book size="small" %}</a>
+            <a href="{{ book.local_path }}">{% include 'snippets/book_cover.html' with book=book cover_class='is-small' %}</a>
         </div>
     </div>
     <div class="column">