Alert users that their messages are very short on compose page

[MyfanwyNixon]

@abimysoc and I have been looking at the message abuse rules and think that 'freeze very short messages' might be more usefully replaced by a pop-up alert on the compose page - maybe with a character count in it. The user would have the option to say 'yeah I know, send it anyway'.

This would hopefully also catch the baffling cases where people submit (and confirm) completely blank messages.