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/user/user.html

@@ -36,7 +36,7 @@
             {% for book in shelf.books %}
             <div class="control">
                 <a href="{{ book.local_path }}">
-                {% include 'snippets/book_cover.html' with book=book size="medium" %}
+                {% include 'snippets/book_cover.html' with book=book cover_class='is-medium' %}
                 </a>
             </div>
             {% endfor %}