Use first_published_at

`details.first_public_at` is deprecated. They should be same, so no
frontend changes are intended.

https://trello.com/c/bMv5Tkgp

Author: tijmenb

app/presenters/content_item/updatable.rb

@@ -14,7 +14,7 @@ def history
     end
 
     def first_public_at
-      content_item["details"]["first_public_at"]
+      content_item["details"]["first_public_at"] || content_item["first_published_at"]
     end
 
     def public_updated_at

test/presenters/content_item/updatable_test.rb

@@ -35,8 +35,8 @@ class << @updatable
       def content_item
         {
           'public_updated_at' => '2002-02-02',
+          'first_published_at' => '2001-01-01',
           'details' => {
-            'first_public_at' => '2001-01-01',
             'change_history' => [
               {
                 'note' => 'note',