Blog: Add Blog To Bitcoin.org

* Adds new page /en/blog

* Adds new feed for blog posts to /en/rss/blog.xml

* Adds "Blog" link to site footer

* Adds three back-dated blog posts to give the blog some initial
  content.

_layouts/base.html

@@ -95,6 +95,7 @@
     {% endcase %}
     <a href="/{{ page.lang }}/{% translate press url %}">{% translate menu-press layout %}</a>
     <a href="/{{ page.lang }}/{% translate about-us url %}">{% translate menu-about-us layout %}</a>
+    <a href="/en/blog">Blog</a>
     </div>
     <div class="footersponsor">
       <div><span>{% translate sponsor layout %}</span> <a href="https://bitcoinfoundation.org/"><img src="/img/brand/bitcoinfoundation.png" alt="Bitcoin Foundation"></a></div>

_layouts/post.html

@@ -0,0 +1,25 @@
+---
+# This file is licensed under the MIT License (MIT) available on
+# http://opensource.org/licenses/MIT.
+
+layout: base
+lang: en
+---
+<div class="subhead-links"><a href="/en/blog">Blog home</a>
+| <a href="/en/rss/blog.xml">Subscribe to RSS feed</a>
+| <a href="https://github.com/bitcoin/bitcoin.org/commits/master/{{page.path|uri_escape}}">Post history</a>
+| <a href="https://github.com/bitcoin/bitcoin.org/issues/new?body=Source%20File%3A%20{{page.path|uri_escape}}%0A%0A">Report issue</a></div>
+
+<div class="post">
+  <header class="post-header">
+    <h1 class="post-title">{{ page.title }}</h1>
+  </header>
+
+  <article class="post-content">
+    {{ content }}
+  </article>
+
+  <p class="post-meta">Posted to the <a href="/en/blog">Bitcoin.org Site
+  Blog</a> on {{ page.date | date: "%d %B %Y" }} {% if page.author %} by {{ page.author }}{% endif %}{% if page.meta %} {{ page.meta }}{% endif %}</p>
+
+</div>