layouts: Fix minor encoding issue with dates

This is similar to other encoding issues we have fixed lately where
we need to make sure Hugo (or Golang's HTML template library) does
not escape our non-ASCII content.

For example, this is how our datetimes end up currently:

    <p class="blog-post-meta"><time datetime="2020-01-16T09:23:20&#43;02:00">Thu Jan 16, 2020</time> by Alan Orth

After printing them with `printf` and filtering them with the Hugo
built-in function `safeHTMLAttr` they look like this:

    <p class="blog-post-meta"><time datetime="2020-01-16T09:23:20+02:00">Thu Jan 16, 2020</time> by Alan Orth

See: https://github.com/gohugoio/hugo/blob/master/tpl/tplimpl/embedded/templates/opengraph.html
See: #111

Author: alanorth

layouts/_default/content.html

@@ -3,7 +3,7 @@
 <article class="blog-post">
   <header>
     <h2 class="blog-post-title" dir="auto"><a href="{{ .Permalink }}">{{ .Title | markdownify }}</a></h2>
-    <p class="blog-post-meta"><time datetime="{{ .Date.Format "2006-01-02T15:04:05Z07:00" }}">{{ .Date.Format $dateFormat }}</time> {{ i18n "authoredBy" }} {{ .Params.author | default .Site.Params.author }}{{ if or (.Params.categories) (.Params.tags) }} {{ i18n "postedIn" }} {{ partial "meta-terms.html" . }}{{ end }}</p>
+    <p class="blog-post-meta"><time {{ .Date.Format "2006-01-02T15:04:05Z07:00" | printf "datetime=%q" | safeHTMLAttr }}>{{ .Date.Format $dateFormat }}</time> {{ i18n "authoredBy" }} {{ .Params.author | default .Site.Params.author }}{{ if or (.Params.categories) (.Params.tags) }} {{ i18n "postedIn" }} {{ partial "meta-terms.html" . }}{{ end }}</p>
   </header>
   {{ .Content }}
 </article> <!-- /.blog-post -->

layouts/_default/single.html

@@ -56,7 +56,7 @@
 <article class="blog-post">
   <header>
     <h2 class="blog-post-title" dir="auto"><a href="{{ .Permalink }}">{{ .Title | markdownify }}</a></h2>
-    <p class="blog-post-meta"><time datetime="{{ .Date.Format "2006-01-02T15:04:05Z07:00" }}">{{ .Date.Format $dateFormat }}</time> {{ i18n "authoredBy" }} {{ .Params.author | default .Site.Params.author }}{{ if or (.Params.categories) (.Params.tags) }} {{ i18n "postedIn" }} {{ partial "meta-terms.html" . }}{{ end }}</p>
+    <p class="blog-post-meta"><time {{ .Date.Format "2006-01-02T15:04:05Z07:00" | printf "datetime=%q" | safeHTMLAttr }}>{{ .Date.Format $dateFormat }}</time> {{ i18n "authoredBy" }} {{ .Params.author | default .Site.Params.author }}{{ if or (.Params.categories) (.Params.tags) }} {{ i18n "postedIn" }} {{ partial "meta-terms.html" . }}{{ end }}</p>
   </header>
   {{ .Content }}
 

layouts/_default/summary.html

@@ -3,7 +3,7 @@
 <article class="blog-post">
   <header>
     <h2 class="blog-post-title" dir="auto"><a href="{{ .Permalink }}">{{ .Title | markdownify }}</a></h2>
-    <p class="blog-post-meta"><time datetime="{{ .Date.Format "2006-01-02T15:04:05Z07:00" }}">{{ .Date.Format $dateFormat }}</time> {{ i18n "authoredBy" }} {{ .Params.author | default .Site.Params.author }}{{ if or (.Params.categories) (.Params.tags) }} {{ i18n "postedIn" }} {{ partial "meta-terms.html" . }}{{ end }}</p>
+    <p class="blog-post-meta"><time {{ .Date.Format "2006-01-02T15:04:05Z07:00" | printf "datetime=%q" | safeHTMLAttr }}>{{ .Date.Format $dateFormat }}</time> {{ i18n "authoredBy" }} {{ .Params.author | default .Site.Params.author }}{{ if or (.Params.categories) (.Params.tags) }} {{ i18n "postedIn" }} {{ partial "meta-terms.html" . }}{{ end }}</p>
   </header>
   {{ .Summary }}
   <a href='{{ .Permalink }}'>{{ i18n "readMore" }}</a>