publish should support POSSEing person-tag reply to Facebook

[tantek]

This is a feature request for POSSEing person-tag reply posts.

I.e. if I post a person-tag reply on my own site, like documented here:

https://indieweb.org/tag-reply#person_tag_reply

where one (or more) of the reply-to URLs is to a Facebook permalink post (likely others's photos, but could be my own that I'm adding person-tags to after I published it), Bridgy Publish (of my person-tag reply) should propagate that person-tag reply to that FB permalink.

This is distinctly different (but may be related to)

• publish should propagate person-tags to POSSE copies #459 (probably most similar to this, which is about propagating person-tags on your own photo post that you're POSSEing)
• backfeed person tags (especially on photos) to the original posts #488
• backfeed responses to mentions/tags #540

[snarfed]

thanks for filing! FB API docs are pretty terse on this, but it looks like we should be able to do it by POSTing to /[photo_id]/tags: https://developers.facebook.com/docs/graph-api/reference/photo/tags/#Creating

[snarfed]

bridgy publish for facebook will disappear soon (#817), so I'm closing open issues for it. sad. feel free to holler if you disagree.