Evaluate SingleFile as alternative serializer

[erikrose]

MAML found https://github.com/mozilla-applied-ml/SingleFile worked on iframes, which we suspect might be one of the reasons freeze-dry (which FathomFox uses) sometimes fails to serialize a page. Explain exactly how FathomFox fails, and see what SingleFile does on the credit-card-form pages from some of these sites:

• www.notebooksbilliger.de
• www.alternate.de
• www.aboutyou.de
• www.hm.com/de
• www.ikea.com/de/de/