Messages Layout with Flexbox API

[jimmcgaw]

Right now messages are laid out in message-box in order to preserve the "auto scroll to bottom on new message" behavior. This behavior would break if floats were not used to position the message elements inside of this container.

Convert the layout of messages to flexbox API, using the margin-right/left: auto in order to achieve the same positioning effect. Look into using flex-direction: column-reverse on the messages container in order to achieve this same behavior without the JavaScript hack currently in place in the message box component.