Don't assume that link domains have a known user

This is an edge case, and they OUGHT to have a user, but if the user was
deleted for a remote link, the field won't be able to resolve and the
user will be unset.

bookwyrm/templates/book/file_links/edit_links.html

@@ -42,7 +42,11 @@
                     <a href="{{ link.url }}" target="_blank" rel="noopener noreferrer">{{ link.url }}</a>
                 </td>
                 <td>
+                    {% if link.added_by %}
                     <a href="{% url 'user-feed' link.added_by.id %}">{{ link.added_by.display_name }}</a>
+                    {% else %}
+                    <em>{% trans "Unknown user" %}</em>
+                    {% endif %}
                 </td>
                 <td>
                     {{ link.filelink.filetype }}
@@ -50,7 +54,7 @@
                 <td>
                     {{ link.domain.name }}
                     <p>
-                        <a href="{% url 'report-link' link.added_by.id link.id %}">{% trans "Report spam" %}</a>
+                        <a href="{% url 'report-link' 0 link.id %}">{% trans "Report spam" %}</a>
                     </p>
                 </td>
                 <td>

bookwyrm/templates/settings/link_domains/link_table.html

@@ -15,7 +15,11 @@
             <a href="{{ link.url }}" target="_blank" rel="noopener noreferrer">{{ link.url }}</a>
         </td>
         <td>
+            {% if link.added_by %}
             <a href="{% url 'settings-user' link.added_by.id %}">@{{ link.added_by|username }}</a>
+            {% else %}
+            <em>{% trans "Unknown user" %}</em>
+            {% endif %}
         </td>
         <td>
             {% if link.filelink.filetype %}