Add "Edit on GitHub" link on normal docs pages

Summary:
<img width="938" alt="screen shot 2016-01-20 at 10 38 23 pm" src="https://cloud.githubusercontent.com/assets/197597/12473376/8f0ac970-bfc6-11e5-9e44-0481e9f250f1.png">
Closes #5454

Reviewed By: svcscm

Differential Revision: D2849235

Pulled By: vjeux

fb-gh-sync-id: 0103f3d5249b1dfc03aa77feb9ae3bb7bbe9c746

Author: vjeux

website/layout/DocsLayout.js

@@ -23,7 +23,14 @@ var DocsLayout = React.createClass({
           <DocsSidebar metadata={metadata} />
           <div className="inner-content">
             <a id="content" />
-            <h1>{metadata.title}</h1>
+            <h1>
+              {metadata.title}
+              <a
+                className="edit-github"
+                href={'https://github.com/facebook/react-native/blob/master/docs/' + metadata.filename}>
+                Edit on GitHub
+              </a>
+            </h1>
             <Marked>{content}</Marked>
             <div className="docs-prevnext">
               {metadata.previous && <a className="docs-prev" href={metadata.previous + '.html#content'}>&larr; Prev</a>}

website/server/convert.js

@@ -52,7 +52,7 @@ function execute() {
     files: [],
   };
 
-  function handleMarkdown(content) {
+  function handleMarkdown(content, filename) {
     var metadata = {};
 
     // Extract markdown metadata header
@@ -78,6 +78,8 @@ function execute() {
       return;
     }
 
+    metadata.filename = filename;
+
     // Create a dummy .js version that just calls the associated layout
     var layout = metadata.layout[0].toUpperCase() + metadata.layout.substr(1) + 'Layout';
 
@@ -105,14 +107,16 @@ function execute() {
     fs.writeFileSync(targetFile, content);
   }
 
-  extractDocs().forEach(handleMarkdown);
+  extractDocs().forEach(function(content) {
+    handleMarkdown(content, null);
+  });
 
   var files = glob.sync(MD_DIR + '**/*.*');
   files.forEach(function(file) {
     var extension = path.extname(file);
     if (extension === '.md' || extension === '.markdown') {
       var content = fs.readFileSync(file, {encoding: 'utf8'});
-      handleMarkdown(content);
+      handleMarkdown(content, path.basename(file));
     }
 
     if (extension === '.json') {