Include 'published' date of posts in Phame Atom feeds

Summary:
Metadata of Phame blog posts includes the 'updated' date of a blog post. Make them also include the original 'published' date.

Patch written by @20after4 from:

https://phabricator.wikimedia.org/rPHAB3de500bfc845759d6da82180df0adfc12f973463

Closes T15686

Test Plan:
* Go to the Atom feed of blog 1 at `/phame/blog/feed/1/` and look at this `<feed>`'s source
* Apply patch and do the same, see additional `<published>` entry just like the `<updated>`
* Edit the last blog post in blog 1, then check Atom feed and verify that the published and updated date stamp differ as expected

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, 20after4, Cigaryno

Maniphest Tasks: T15686

Differential Revision: https://we.phorge.it/D25490

src/applications/phame/controller/blog/PhameBlogFeedController.php

@@ -71,6 +71,11 @@ final class PhameBlogFeedController extends PhameBlogController {
         '<author><name>%s</name></author>',
         $bloggers[$post->getBloggerPHID()]->getFullName());
 
+      $content[] = phutil_tag(
+        'published',
+        array(),
+        date('c', $post->getDatePublished()));
+
       $content[] = phutil_tag(
         'updated',
         array(),