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/import_status.html

@@ -125,7 +125,7 @@
             <td>
                 {% if item.book %}
                 <a href="/book/{{ item.book.id }}">
-                {% include 'snippets/book_cover.html' with book=item.book size='small' %}
+                {% include 'snippets/book_cover.html' with book=item.book cover_class='is-small' %}
                 </a>
                 {% endif %}
             </td>