Default theme implementation to present feeds as list items.
Each iteration generates a single feed source or category.
Available variables:
template_preprocess_aggregator_summary_items()
- {#
 - /**
 -  * @file
 -  * Default theme implementation to present feeds as list items.
 -  *
 -  * Each iteration generates a single feed source or category.
 -  *
 -  * Available variables:
 -  * - title: Title of the feed or category.
 -  * - summary_list: Unordered list of linked feed items generated through
 -  *   theme_item_list().
 -  * - source_url: URL to the local source or category.
 -  *
 -  * @see template_preprocess()
 -  * @see template_preprocess_aggregator_summary_items()
 -  *
 -  * @ingroup themeable
 -  */
 - #}
 - <h3>{{ title }}</h3>
 - {{ summary_list }}
 - <div class="links">
 -   <a href="{{ source_url }}">{{ 'More<span class="element-invisible"> posts about %title</span>'|t({ '%title': title }) }}</a>
 - </div>