cover: Use book-cover as component:

- Avoid specifying context-dependent values in CSS for components. Those values can be defined by the context calling the component.
- Use `<figure>` with optional caption.
- Reduce redundant markup.
- Allow more variables to be passed to the book-cover (image path and class for the container).
- Hide the book cover to screen readers.

bookwyrm/templates/discover/small-book.html

@@ -1,12 +1,24 @@
+
 {% load bookwyrm_tags %}
 {% load i18n %}
+
 {% if book %}
-<a href="{{ book.local_path }}">{% include 'snippets/book_cover.html' with book=book %}</a>
-{% include 'snippets/stars.html' with rating=book|rating:request.user %}
+    {% with book=book %}
+        <a
+            href="{{ book.local_path }}"
+        >{% include 'snippets/book_cover.html' with size="small" %}</a>
 
-<h3 class="title is-6"><a href="/book/{{ book.id }}">{{ book.title }}</a></h3>
-{% if book.authors %}
-<p class="subtitle is-6">{% trans "by" %} {% include 'snippets/authors.html' with book=book %}</p>
-{% endif %}
+        {% include 'snippets/stars.html' with rating=book|rating:request.user %}
 
+        <h3 class="title is-6">
+            <a href="/book/{{ book.id }}">{{ book.title }}</a>
+        </h3>
+
+        {% if book.authors %}
+            <p class="subtitle is-6">
+                {% trans "by" %}
+                {% include 'snippets/authors.html' %}
+            </p>
+        {% endif %}
+    {% endwith %}
 {% endif %}