Stretch keyboard shortcut dialog

adds a `modal_stretch` CSS class to fill the screen for large dialogs (e.g. keyboard shortcuts). This performs better on both large and small screens than regular dialogs with a fixed max-height.

I had to remove an old `width: 100%` workaround for Firefox that @jdfreder assured me is no longer needed with the new `display: flex`.

small screen: 
![small](https://cloud.githubusercontent.com/assets/151929/2543196/b94e86b4-b5ef-11e3-880e-d1d01c1f75ff.png)

large screen: 
![large](https://cloud.githubusercontent.com/assets/151929/2543204/c7ea8d44-b5ef-11e3-90ca-0004a252b00b.png)

closes #5413.