Split fixtures into DCRArticle and FEArticle

[arelra]

Completed by #10864

As part of the gen-fixtures script we currently output articles using the enhanced DCRArticle type. However Playwright tests expect the FEArticle type to POST to DCR.

This currently works as the DCRArticle type extends the FEArticle type but from #10801 the types will diverge.

Re-generating the fixtures has highlighted a few issues:

• Auto generated links regression
• Missing subnav on DecideLayout:Web Standard Standard News Light: Subnav missing from articles with tag us-news/us-politics tag #11005
• Incorrect subnav on DecideLayout:Web Liveblog With No Key Events: Split Frontend and DCR fixtures #10864 (comment)
• Lightbox diffs