add blog link to footer

src/templates/_footerlinks.html

···
   <a href="{{ url_for('page_terms') }}">terms of service</a>
   <span>·</span>
   <a href="https://tangled.org/@ligo.at/core" target="_blank">source code</a>
+  <span>·</span>
+  <a href="https://blog.ligo.at">blog</a>
 </p>