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/feed/feed_layout.html

@@ -37,7 +37,7 @@
                                         aria-label="{{ book.title }}"
                                         aria-selected="{% if active_book == book.id|stringformat:'d' %}true{% elif shelf_counter == 1 and forloop.first %}true{% else %}false{% endif %}"
                                         aria-controls="book-{{ book.id }}">
-                                        {% include 'snippets/book_cover.html' with book=book size="medium" %}
+                                        {% include 'snippets/book_cover.html' with book=book cover_class='is-medium' %}
                                     </a>
                                 </li>
                                 {% endfor %}