Fix date format in `datetime` attribute

Previously, the date format in this attribute for posts was invalid.
This caused local date rendering to fail in Firefox. This fixes that.

Closes #253

templates/chorus-collection-post.tmpl

@@ -58,7 +58,7 @@ body#post header {
 		{{if .Suspended}}
 			{{template "user-suspended"}}
 		{{end}}
-		<article id="post-body" class="{{.Font}} h-entry">{{if .IsScheduled}}<p class="badge">Scheduled</p>{{end}}{{if .Title.String}}<h2 id="title" class="p-name{{if $.Collection.Format.ShowDates}} dated{{end}}">{{.FormattedDisplayTitle}}</h2>{{end}}{{if $.Collection.Format.ShowDates}}<time class="dt-published" datetime="{{.Created}}" pubdate itemprop="datePublished" content="{{.Created}}">{{.DisplayDate}}</time>{{end}}<div class="e-content">{{.HTMLContent}}</div></article>
+		<article id="post-body" class="{{.Font}} h-entry">{{if .IsScheduled}}<p class="badge">Scheduled</p>{{end}}{{if .Title.String}}<h2 id="title" class="p-name{{if $.Collection.Format.ShowDates}} dated{{end}}">{{.FormattedDisplayTitle}}</h2>{{end}}{{if $.Collection.Format.ShowDates}}<time class="dt-published" datetime="{{.Created8601}}" pubdate itemprop="datePublished" content="{{.Created}}">{{.DisplayDate}}</time>{{end}}<div class="e-content">{{.HTMLContent}}</div></article>
 
 		{{ if .Collection.ShowFooterBranding }}
 		<footer dir="ltr">

templates/collection-post.tmpl

@@ -62,7 +62,7 @@
 		{{if .Suspended}}
 			{{template "user-suspended"}}
 		{{end}}
-		<article id="post-body" class="{{.Font}} h-entry {{if not .IsFound}}error-page{{end}}">{{if .IsScheduled}}<p class="badge">Scheduled</p>{{end}}{{if .Title.String}}<h2 id="title" class="p-name{{if $.Collection.Format.ShowDates}} dated{{end}}">{{.FormattedDisplayTitle}}</h2>{{end}}{{if $.Collection.Format.ShowDates}}<time class="dt-published" datetime="{{.Created}}" pubdate itemprop="datePublished" content="{{.Created}}">{{.DisplayDate}}</time>{{end}}<div class="e-content">{{.HTMLContent}}</div></article>
+		<article id="post-body" class="{{.Font}} h-entry {{if not .IsFound}}error-page{{end}}">{{if .IsScheduled}}<p class="badge">Scheduled</p>{{end}}{{if .Title.String}}<h2 id="title" class="p-name{{if $.Collection.Format.ShowDates}} dated{{end}}">{{.FormattedDisplayTitle}}</h2>{{end}}{{if $.Collection.Format.ShowDates}}<time class="dt-published" datetime="{{.Created8601}}" pubdate itemprop="datePublished" content="{{.Created}}">{{.DisplayDate}}</time>{{end}}<div class="e-content">{{.HTMLContent}}</div></article>
 
 		{{ if .Collection.ShowFooterBranding }}
 		<footer dir="ltr"><hr><nav><p style="font-size: 0.9em">{{localhtml "published with write.as" .Language.String}}</p></nav></footer>

templates/include/posts.tmpl

@@ -21,10 +21,10 @@
 			{{end}}
 		{{end}}
 	</h2>
-	{{if $.Format.ShowDates}}<time class="dt-published" datetime="{{.Created}}" pubdate itemprop="datePublished" content="{{.Created}}">{{if not .Title.String}}<a href="{{$.CanonicalURL}}{{.Slug.String}}" itemprop="url">{{end}}{{.DisplayDate}}{{if not .Title.String}}</a>{{end}}</time>{{end}}
+	{{if $.Format.ShowDates}}<time class="dt-published" datetime="{{.Created8601}}" pubdate itemprop="datePublished" content="{{.Created}}">{{if not .Title.String}}<a href="{{$.CanonicalURL}}{{.Slug.String}}" itemprop="url">{{end}}{{.DisplayDate}}{{if not .Title.String}}</a>{{end}}</time>{{end}}
 {{else}}
 <h2 class="post-title" itemprop="name">
-	{{if $.Format.ShowDates}}<time class="dt-published" datetime="{{.Created}}" pubdate itemprop="datePublished" content="{{.Created}}"><a href="{{if not $.SingleUser}}/{{$.Alias}}/{{.Slug.String}}{{else}}{{$.CanonicalURL}}{{.Slug.String}}{{end}}" itemprop="url" class="u-url">{{.DisplayDate}}</a></time>{{end}}
+	{{if $.Format.ShowDates}}<time class="dt-published" datetime="{{.Created8601}}" pubdate itemprop="datePublished" content="{{.Created}}"><a href="{{if not $.SingleUser}}/{{$.Alias}}/{{.Slug.String}}{{else}}{{$.CanonicalURL}}{{.Slug.String}}{{end}}" itemprop="url" class="u-url">{{.DisplayDate}}</a></time>{{end}}
 	{{if $.IsOwner}}
 		{{if not $.Format.ShowDates}}<a class="user hidden action" href="{{if not $.SingleUser}}/{{$.Alias}}/{{.Slug.String}}{{else}}{{$.CanonicalURL}}{{.Slug.String}}{{end}}">view</a>{{end}}
 		<a class="user hidden action" href="/{{if not $.SingleUser}}{{$.Alias}}/{{end}}{{.Slug.String}}/edit">edit</a>