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/snippets/search_result_text.html

@@ -1,6 +1,6 @@
 {% load i18n %}
 <div class="columns is-mobile is-gapless">
-    {% include 'snippets/book_cover.html' with book=result container_class='column' img_path=false %}
+    {% include 'snippets/book_cover.html' with book=result cover_class='column' img_path=false %}
 
     <div class="column is-10 ml-3">
         <p>