Use reverse lookups for urls

bookwyrm/templates/discover/large-book.html

@@ -17,7 +17,7 @@
 
             <div class="column mt-3-mobile ml-3-tablet">
                 <h3 class="title is-5">
-                    <a href="/book/{{ book.id }}">{{ book.title }}</a>
+                    <a href="{{ book.local_path }}">{{ book.title }}</a>
                 </h3>
 
                 {% if book.authors %}

bookwyrm/templates/discover/small-book.html

@@ -11,7 +11,7 @@
         {% include 'snippets/stars.html' with rating=book|rating:request.user %}
 
         <h3 class="title is-6">
-            <a href="/book/{{ book.id }}">{{ book.title }}</a>
+            <a href="{{ book.local_path }}">{{ book.title }}</a>
         </h3>
 
         {% if book.authors %}