refactor: inline back_link.html into post layout

Author: Agil Mammadov

_includes/back_link.html

@@ -1,7 +1,7 @@
 ---
 layout: default
 ---
-{%- include back_link.html -%}
+<a href="{{ '/' | relative_url }}">{{ site.theme_config.back }}</a>
 
 <article>
   <span class="post-meta">