Fix #8683 Relative URLs are not converted to absolute URLs in atom feed

[schubtob]

Issue #8683

This PR fixes a problem with relative URLs in atom feeds.
Prior to this commit relative URLs were only converted to absolute URLs in the summary tag and not in the content tag.

With the proposed change, relative URLs are also converted to absolute URLs in the content tag.

How to test

1. Create an article including relative URLs and/or embedded images.
2. Access the atom feed URL that includes this article.
3. Notice that the relative URLs are converted to absolute URLs only in the summary tag and not in the content tag.
4. Apply this PR
5. Access the atom feed again and notice that relative URLs are converted in both the summary and the content tag.

[andrepereiradasilva]

I have tested this item ✅ successfully on 755372b

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[anibalsanchez]

I have tested this item ✅ successfully on 755372b

Test OK

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[brianteeman]

Setting RTC - thanks

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