Enable checkbox clear for ie

Fixes this: https://www.pivotaltracker.com/story/show/68701208

@Edds pointed out that ideally we should be enhancing by feature not blanket off/on with JS. This PR enables the following features in ie 6/7:

- clearing all contained checkboxes via the clear link:
- resizing checkbox containers on load so they fit snugly where possible

 Still disabled:
- Opening and closing containers as performance isn't good enough, setting them to be open all the time is better.