Make s3 upload event optionally take thread & group id
Make s3 upload event optionally take thread & group id

Fixes #170, as drag-and-drop uploading expects to pass a map of url,
group-id, and thread-id, while the even was expecting just the URL
string, with the thread & group ids being set separately.

rafd/braid,braidchat/braid,rafd/braid,braidchat/braid