Replaced admin help popup with iframe overlay

1. Better experience for non-windowed browsers (smartphones).
2. Help page opens without delay because it avoids the browser’s popup protection.
3. Overlay resizes with the viewport, good all the way down to mobile sizes.
4. Overlay can be closed by tapping the periphery as well as using the X gadget.

I've also moved this behaviour to the admin theme's JavaScript – I think this behaviour is a choice the theme maker should be able to control.

Screenshot of the new help overlay:
![screen shot 2014-06-22 at 10 46 28](https://cloud.githubusercontent.com/assets/4855211/3351157/d77cfc06-f9f6-11e3-90f2-230902088817.png)