Add support for Facebook original post discovery

mf2/shim supports parsing of facebook post permalink pages, as fortunately they’re transmitted as real HTML if you provide a real-looking user-agent string.

Apply the same trailing link discovery as to twitter, along with any other silo-specific heuristics which seem suitable.

Start by locating the existing Twitter trailing-link discovery and the Facebook handling in mf2/shim; inspect how Facebook permalink pages are parsed with a real-looking user-agent. Done means Facebook pages receive equivalent trailing-link discovery and suitable silo-specific heuristics, with relevant tests or fixtures updated if present.

Written by the indexing model from the issue text.