Add ability to preview Mobile Theme

* Add an Emulate Mobile section on the Preview as User panel.
* If enabled, set the mobile theme to the View object.
* Add a `resolution-width` input to change the `max-width` value of the container.

![emulate_mobile](https://cloud.githubusercontent.com/assets/514294/5514034/cb745e4c-8860-11e4-88ed-026312fc99b2.png)