Use site title in header, not context's title

Depending on the context the ".Title" could be the post or page's
title, but here we actually want the site's title.

layouts/partials/header.html

@@ -1,6 +1,6 @@
 <div class="blog-header">
   <div class="container">
-    <h1 class="blog-title">{{ .Title | safeHTML }}</h1>
+    <h1 class="blog-title">{{ .Site.Title | safeHTML }}</h1>
     {{ if .Site.Params.description }}<p class="lead blog-description">{{ .Site.Params.description | safeHTML }}</p>{{ end }}
   </div>
 </div>