allow user input of locations

[sethvincent]

two possible approaches:

1. create the form in this repository, and send it to the flatsheet instance with a POST request to the api
2. create the form in the seward.flatsheet.io repository, and embed an iframe with that form here.

The 2nd option is tempting, because currently api POST requests require authentication with username & password, which makes zero sense for including in a browser app. So I think I'm going with option 2.

That means this issue should really be at https://github.com/civicmakerlab/seward.flatsheet.io/issues

I'll add it there.