Use id instead of name for TableOfContentsFilter

Closes #135

I also added the [aria-hidden](http://www.w3.org/TR/wai-aria/states_and_properties#aria-hidden) attribute, which tells assistive devices that this element is not actually visible. We already set this on GitHub, so I thought it would make sense to be included upstream.

/cc @jch @josh 


> aria-hidden="true

cool, should work nicely with this accessibly detecter

https://github.com/github/github/blob/master/app/assets/javascripts/staff/logging/accessibility.coffee#L59