Support mobile sizes

We don't currently display or allow commenting on mobile sizes.

These changes will allow existing comments to be viewed at mobile sizes and to
be able to add to an existing thread.

We don't currently support being able to add comments on a paragraph where none
are yet, as we don't support touch events instead of hover.

![comments](https://cloud.githubusercontent.com/assets/5015/4685109/7a59bcd8-5637-11e4-8343-55e55be1a588.png)

0px -> 0