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/book/editions.html

@@ -16,7 +16,7 @@
     <div class="columns">
         <div class="column is-2">
             <a href="/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>
         </div>
         <div class="column is-7">