Convert format_rss_channel() and format_rss_item() to twig templates

Problem/Motivation

Prompted by this conversion:
http://drupal.org/node/1896060#comment-7274838

This is in an attempt to allow those output tags in
format_rss_channel()
format_rss_item()

to be rendered as twig files. The issue above is for aggregator module but this would be a template needed by both node_feed(), which is used by taxonomy and the aggregator module.

Proposed resolution

Create a new theme function for these format functions, and use the theme for theme_aggregator_page_rss() and node_feed()

API changes

Removes these functions and replaces them with Twig templates:

format_rss_channel()
format_rss_item()

Related Issues

#401584: Convert format_rss_items(), format_rss_channel() and format_xml_elements() to use SimpleXML
#1896060: aggregator.module - Convert PHPTemplate templates to Twig