Cannot create messages for selected call

[maser]

I can only create messages that go to "All Users". If I select one of the other options (e.g. "All Users for selected Call") and select a call (hint: only open calls are shown), a validation error is shown on the "Recipients" field.

I went back to bc8e848 and could reproduce it, so I believe we did not break this in recent changes.

[bitboxer]

@maser this is not important for the start of the CFP, but we should have this working to send them a "the CFP is closed, thanks for sending yours" message when we are through