Fix links in blog Atom feed (#421)

The root URL was missing the baseURL portion. This is fine when a site is hosted on a custom URL such as docusaurus.io, because the generated URL will be valid (docusaurus.io/blog), but it breaks when the site is hosted on GitHub Pages (the React Native feed was using facebook.github.io/blog as the rootURL).

* Fix link to header image

lib/server/feed.js

@@ -16,8 +16,9 @@ const CWD = process.cwd();
 const siteConfig = require(CWD + '/siteConfig.js');
 
 const blogFolder = path.resolve('../blog/');
-const blogRootURL = siteConfig.url + '/blog';
-const jestImage = siteConfig.url + '/' + siteConfig.headerIcon;
+const blogRootURL = siteConfig.url + siteConfig.baseUrl + 'blog';
+const siteImageURL =
+  siteConfig.url + siteConfig.baseUrl + siteConfig.headerIcon;
 
 /****************************************************************************/
 
@@ -46,7 +47,7 @@ module.exports = function(type) {
       ' news and events.',
     id: blogRootURL,
     link: blogRootURL,
-    image: jestImage,
+    image: siteImageURL,
     copyright: siteConfig.copyright,
     updated: new Date(MetadataBlog[0].date),
   });