Indicating whether posts have already been responded to

The Microsub client needs to know whether a post has already been liked/replied/bookmarked/etc by the user in order to display a the appropriate interface buttons. Similarly the client should allow the user to delete the "like" post if it's been posted already, so it will need to know the URL of the like.

One way to accomplish this for external Microsub servers is the Microsub server can subscribe to your own site and index all the "like" posts it finds, and match those up. If your site is also your micropub and microsub server then all this is internal to the system and doesn't need to be specified.

The issue names no files, tests, or implementation entry points. Start by clarifying how a Microsub client should learn whether a post was liked, replied to, or bookmarked, and how it should obtain the URL of an existing like. Done requires an agreed specification for external Microsub servers, while the internal-server case is explicitly out of scope.

Written by the indexing model from the issue text.