feat: support hiding the modification date of a post (#1020)

There is the current ability to set `last_modified_at` in the front matter of post and use that datetime instead of the built in logic.
New feature in post.html:  if the modified date time matches the date of the post, then don't show "Updated xxxx"

_includes/update-list.html

@@ -7,7 +7,7 @@
 {% assign all_list = '' | split: '' %}
 
 {% for post in site.posts %}
-  {% if post.last_modified_at %}
+  {% if post.last_modified_at and post.last_modified_at != post.date %}
     {% capture elem %}
       {{- post.last_modified_at | date: "%Y%m%d%H%M%S" -}}::{{- forloop.index0 -}}
     {% endcapture %}

_layouts/post.html

@@ -21,7 +21,7 @@ tail_includes:
     </span>
 
     <!-- lastmod date -->
-    {% if page.last_modified_at %}
+    {% if page.last_modified_at and page.last_modified_at != page.date %}
     <span>
       {{ site.data.locales[lang].post.updated }}
       {% include datetime.html date=page.last_modified_at tooltip=true lang=lang %}