layout: improve list of post taxonomy terms

Wraps the flex of the taxonomy term list to prevent the content being
larger than small screens. Also improves the solution for the coma
separator, to get a coma after a term (and not before), and not after
the last item of the list.

Closes #10

Co-Authored-by: Igor Milhit <igor@milhit.ch>

assets/scss/_post.scss

@@ -25,6 +25,7 @@
     display: flex;
     > ul {
       display: flex;
+      flex-wrap: wrap;
       padding: 0;
       white-space: pre;
     }
@@ -35,6 +36,8 @@
   }
 }
 
-.post-taxonomies + .post-taxonomies:before {
+.post-taxonomies:not(:last-child) {
+  &::after {
     content: ", ";
   }
+}