Fix atom.xml feed errors

[stsimb]

• Add missing date_to_xmlschema and xml_escape Liquid filters
• Fix invalid post.id reference (changed to post.url)
• Remove redundant xml_escape from CDATA content block
• Keep xml_escape for title element (required outside CDATA)

Should now validate with https://validator.w3.org/feed/check.cgi

[stsimb]

@codegaze I know this may seem like an unusual PR, but my RSS reader cannot use your existing atom feed.

The main problem is that each article requires a unique id, but post.id is empty in your case, and thus the id field is the same in all entries (site.url) and the feed is invalid.

Changing post.id to post.url satisfies the uniqueness requirement and will render the feed usable.

The next problem would be the missing filters and escaping twice the content block, so I've added quick fixes for these too.

Thank you for posting meaningful content in your blog, and please consider accepting this PR to make your feed usable to the few rss readers left nowadays :-)