Limit Atom feeds to 20 items by default, add pagination by max_id

(But there are no Atom feed pagination elements yet)

app/views/accounts/show.atom.ruby

@@ -15,7 +15,7 @@ Nokogiri::XML::Builder.new do |xml|
     link_hub       xml, Rails.configuration.x.hub_url
     link_salmon    xml, api_salmon_url(@account.id)
 
-    @entries.order('id desc').each do |stream_entry|
+    @entries.each do |stream_entry|
       entry(xml, false) do
         include_entry xml, stream_entry
       end