layouts/partials/sidebar.html: Use .Site.RegularPages instead of .Site.Pages

As of Hugo 0.18 .Site.Pages now returns all pages, including pages,
home, taxonomies, etc, and you are expected to filter them by their
"kind". There is a new variable .Site.RegularPages which returns a
range of "regular" pages like in pre-0.18 Hugo.

In this instance we are limiting the range to pages with the type
"post", so our current behavior works, but I'd rather be consistent
with other ranges we're using, like on the homepage and taxonomy
list pages.

See: https://github.com/spf13/hugo/releases/tag/v0.18

layouts/partials/sidebar.html

@@ -12,7 +12,7 @@
   <section class="sidebar-module">
     <h4>{{ i18n "recentPosts" }}</h4>
     <ol class="list-unstyled">
-      {{ range first $num_recent_posts (where .Site.Pages "Type" "post") }}
+      {{ range first $num_recent_posts (where .Site.RegularPages "Type" "post") }}
       <li><a href="{{.RelPermalink}}">{{.Title | markdownify }}</a></li>
       {{ end }}
     </ol>