microformats2 h-entry support

[delan]

• https://microformats.org/wiki/h-entry
• https://nex-3.com/blog/reblogging-posts-with-h-entry/ by @nex3

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• [x] render posts with h-entry
• [x] import posts with h-entry
  • [x] import from url without modifying the .e-content
  • [x] rewrite relative urls (microformats/microformats2-parsing#38)
  • [x] cache embedded images as attachments
  • [x] #18

[delan]

i think this now works well enough to close, splitting #18 into its own issue.