add delete post route

app/templates/posts/edit.html

@@ -1,7 +1,12 @@
+{%- from 'common/macros.html' import subheader -%}
 {%- from 'common/macros.html' import full_post with context -%}
 {%- extends 'base.html' -%}
 {%- block title -%}editing a post{%- endblock -%}
 {%- block content -%}
+  {%- set nav -%}
+    <a href="{{get_post_url(post.id, _anchor=true)}}">&larr; Back to thread</a>
+  {%- endset -%}
+  {{ subheader("Editing your post", nav)}}
   {%- for post in context_prev -%}
   <div class="post plank">{{- full_post(post=post, show_toolbar=false, show_reactions=false) -}}</div>
   {%- endfor -%}