Jekyll: Add support for fixing the URLs in <img srcset> attribute

[markstos]

mg-jekyll-export currently handles updating URLS in `<Img> "src" attributes. It would be great of the "srcset" attribute was handled as well.

In the #506 PR, there's now an HTML processing function that takes an old blogs's HTML input and produces cleaned / migrated HTML as output. This function is not specific to Jekyll at all. A lot of other import sources could use the same kind of HTML processing, so it seems like it makes sense to have that logic to a shared location, the way that mg-linkfixer is handled. The HTML processing code in the Jekyll export tool is mostly concerned with link processing, but it also handles some other kinds of bad HTML, otherrwise I would suggest putting the code in mg-linkfixer.