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/edit_book.html

@@ -170,7 +170,7 @@
             <h2 class="title is-4">{% trans "Cover" %}</h2>
             <div class="columns">
                 <div class="column is-narrow">
-                    {% include 'snippets/book_cover.html' with book=book size="small" %}
+                    {% include 'snippets/book_cover.html' with book=book cover_class='is-small' %}
                 </div>
                 <div class="column is-narrow">
                     <div class="block">