Standardize modal/overlay JS
This long-delayed commit finally brings up-to-date a standardized modal overlay for use in bedrock. Refs bug 868132. It's implemented on:
- firefox/partners
- firefox/geolocation
- firefox/os

It's not implemented on the 2011 annual report as this was going to involve more work and it's basically an archived page. That said, I can update it if necessary.