Google Mail feeds have been observed with this `issued` tag.
Also change to test first the strings conforming to the official
specifications.

https://datatracker.ietf.org/doc/html/rfc4287
https://validator.w3.org/feed/docs/atom.html

Fix Validate button to pass correctly the feed's URL string to the
validator page (https://validator.w3.org/feed/check.cgi) when the feed
URL itself contains a query.
Problem seen on Flickr's feeds, as well as on rss-bridge.org or YouTube
feeds.

Also: do not change inconsiderately the content of the URL field