UI and backend to allow players to submit/update chat messages when it's not their turn

* Each player gets one chat message per attack --- if you don't say anything before your attack, you can say something afterwards.  The most recent message can be edited or deleted until the opponent takes their turn.
* Currently, the UI only allows chat to be submitted while waiting for the other player to attack (not during swing die setting, focus turndown, etc).  I think this is pretty close to what the old site did, and should be an improvement over the current behavior.
* Fixes #338
* Jenkins: http://jenkins.buttonweavers.com:8080/job/buttonmen-cgolubi1/229/