Correct checkbox id
Add full ul/li to checkbox list

Added full ul/li back to allow semantically correct nesting (useful for javascript checking/unchecking of nested checkboxes).
I know uls/li used in this way lead to additional (and unhelpful) screen reader chatter, can't think of a better way at the moment but if you can PLEASE TELL ME.
Initial go at checkbox behaviours

- Probably doesn't want to go in application.js ultimately
- Needs ie6 exclusion
- Needs all the other stuff that whitehall has.