Feed urls

Feedly is broken and doesn't understand base URLs. We assumed that post
bodies were still relative to the post directory and not the feed
itself. Now absolute URLs are used for both self links and embedded
images. Fixes ocaml/platform-blog#12.

Hopefully feedly et al understand `xml:base`. If not, we can map attributes like in the commented section. Will merge, re-run, and test with feedly to see how bad it is.