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/discover/large-book.html

@@ -8,7 +8,7 @@
             <div class="column">
                 <a
                     href="{{ book.local_path }}"
-                >{% include 'snippets/book_cover.html' with size="large" %}</a>
+                >{% include 'snippets/book_cover.html' with cover_class='is-large' %}</a>
 
                 {% include 'snippets/stars.html' with rating=book|rating:request.user %}
             </div>