on keypress on the form it updates a byte counter

* adds a <small> tag with an approximation of the bytes used by the
  message
* the overhead tends to be ~300 bytes so that's why the counter says of
  ~724 bytes

There might be a better way to do this? This seemed pretty straight
forward though. Suggestions are welcome!

I thought about adding a model because it seems like the mercury-way but it seemed a little bit of overkill, thoughts?

GIF:

http://g.recordit.co/DEjreG7bEj.gif