Standardize modal/overlay JS

This long-delayed commit finally brings up-to-date a standardized modal overlay for use in bedrock. 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It's not implemented on the 2011 annual report as this was going to involve more work and it's basically an archived page.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.

It may need a little more testing, and there's one outstanding TODO for figuring out the best way to translate the "Close" text for pages with the modal window included.