Improve alignment of topic actions dropdown

If the footer stretches to two lines (should only happen on mobile and
when there are new comments), this aligns the dropdown button more
towards the bottom of the topic, which looks better.

tildes/scss/modules/_topic.scss

@@ -75,6 +75,7 @@
 
   .topic-actions {
     grid-area: actions;
+    align-self: end;
   }
 }