uploads: add data attributes to thumbnail html.

Add data attributes to thumbnails on the /uploads, /upload_media_assets,
and /media_assets pages. Add a `data-is-posted` attribute for styling
thumbnails based on whether they've already been posted.

app/helpers/application_helper.rb

@@ -317,7 +317,7 @@ module ApplicationHelper
     end
   end
 
-  def data_attributes_for(record, prefix, attributes)
+  def data_attributes_for(record, prefix = "data", attributes = record.html_data_attributes)
     attributes.map do |attr|
       if attr.is_a?(Array)
         name = attr.map {|sym| sym.to_s.dasherize.delete("?")}.join('-')