allow location of posts to be set to a checkin post

[aaronpk]

For food posts like this, the location of that should actually indicate that it was at The Farmer's Union which is a checkin post on my site.

Once Teacup can fetch recent checkins and prompt me to choose one in the interface, then I can store that post as the location of the food post.

I probably want it to look like this:

where the text links to the checkin.

This is different from the way location currently works, since currently the location is never a link. I will likely store the location as an h-entry rather than h-adr, so that it can include the URL as well as the venue details.

[aaronpk]

Incidentally, this would also end up sending a webmention to the checkin, which would show the food photo as a comment!