Opt-in to syndication on a per-post/-event basis
As part of jvt.me#409, we want to selectively syndicate to external sites less often than originally thought, only adding the metadata if required. This means that the content will be marked up with an array of sites and their link text to be shown. Also, don't even show the `#syndication-targets` container unless we're actually syndicating, so HTML-Proofer knows when to complain about links not being set.
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