Re-style Blog, Add Newsletter Signup Form

Summary:
The goal of this PR is to place greater emphasis on the blog as a destination.

The dark Hero from the landing page is now present in the blog as well, and the content is front and center.

The sidebar has been removed. It is not necessary to show a list of recent blog posts in the sidebar when the blog landing page shows the same number of posts along with short excerpts.

The prev/next links have swapped positions, and will now display "Older posts" and "Newer posts".

The excerpts have been stripped of formatting and they are now consistent across the blog landing page and the OpenGraph metatags. Fixes #10597.

A signup form for the new React Native newsletter has been added to the footer.

Newsletter signup form in footer:

![screencapture-localhost-8079-react-native-1477944030909](https://cloud.githubusercontent.com/assets/165856/19869614/4bb035aa-9f6a-11e6-9b8e-e0333417f423.png)

Blog landing page:

![screencapture-localhost-8079-r
Closes https://github.com/facebook/react-native/pull/10660

Differential Revision: D4117034

Pulled By: bestander

fbshipit-source-id: 215f966008fdf5c8870ed28d92384034a0d23c39

website/core/BlogPostHeader.js

@@ -12,6 +12,7 @@
 'use strict';
 
 var React = require('React');
+var BlogPostDate = require('BlogPostDate');
 
 var BlogPostHeader = React.createClass({
   render: function() {
@@ -47,7 +48,7 @@ var BlogPostHeader = React.createClass({
             {post.author}
           </a>
           {' — '}
-          <span className="date">{this.props.postedOnDate}</span>
+          <BlogPostDate post={post} />
         </h4>
         <h1 className="entry-title">{title}</h1>
       </header>