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/lists/list_items.html

@@ -8,11 +8,19 @@
                     <a href="{{ list.local_path }}">{{ list.name }}</a> <span class="subtitle">{% include 'snippets/privacy-icons.html' with item=list %}</span>
                 </h4>
             </header>
-            <div class="card-image is-flex is-clipped">
-                {% for book in list.listitem_set.all|slice:5 %}
-                <a href="{{ book.book.local_path }}">{% include 'snippets/book_cover.html' with book=book.book size="small" %}</a>
-                {% endfor %}
-            </div>
+
+            {% with list_books=list.listitem_set.all|slice:5 %}
+                {% if list_books %}
+                    <div class="card-image columns is-mobile is-gapless is-clipped has-height">
+                        {% for book in list_books %}
+                            <a class="column is-narrow" href="{{ book.book.local_path }}">
+                                {% include 'snippets/book_cover.html' with book=book.book cover_class='is-small' aria='show' %}
+                            </a>
+                        {% endfor %}
+                    </div>
+                {% endif %}
+            {% endwith %}
+
             <div class="card-content is-flex-grow-0">
                 <div {% if list.description %}title="{{ list.description }}"{% endif %}>
                 {% if list.description %}