Add external url into feed

[arash77]

@dannon
That would be great to add external_url and source_blog into the feeds that looks like this:

<entry>
<title type="html">
<![CDATA[ The AquaINFRA Virtual Research Environment ]]>
</title>
<id>https://galaxyproject.org/news/2024-07-29-galaxy-aquainfra/</id>
<link href="https://galaxyproject.org/news/2024-07-29-galaxy-aquainfra/"/>
<updated>2024-07-29T00:00:00.000Z</updated>
<summary type="html">
<![CDATA[ Design rationale for using the Galaxy platform ]]>
</summary>
<author>
<name>Markus Konkol</name>
</author>
<published>2024-07-29T00:00:00.000Z</published>
</entry>

Originally requested by @bgruening in usegalaxy-eu/galaxy-social#87 (comment)

[itisAliRH]

I tried adding this information to the feed result but found it probably impossible because the atom has some limitations, such as field names and data types (RFC4287). The current configuration is also not correct. The description added here is not represented in the output as it is not an atom field.