add "reacji" discovery with reply fallback, consider like fallback

[tantek]

Per 3+ publishers, consider adding reacji discovery:
https://indieweb.org/reacji#IndieWeb_Examples
currently one documented reacji consuming code, need two more.

Like any response, it works with a "reply" fallback. In the case of reacji that indicate a positive response to the post itself (like a thumbs-up 👍 , smiley 😄 , etc), it may also be worth considering recommending publishing and detecting a "like" as fallback.

[tantek]

Unclear if there are 3+ consuming implementations yet (looks like 2 for sure). Thus postponing at this time.