create messages on click in messages pool

OK, @jh-bate, the message images are being added now, but they will disappear if you scroll away and scroll back until I get from you the actual creation event with the ID.

I think we'll also need to have a way of deleting a message thread from the modal, since with this interaction it may be pretty easy to accidentally create a new message.

This isn't ready for merge quite yet given comments above, just starting the PR for conversation and progress reports.