Search Results page -- make nested checkboxes (subfacets) keyboard-accessible

[lhenze]

From NYU A11y Review:

Under the Resource Types heading, the Publications menu is not constructed in a way that the keyboard-only user can access the sub-filter elements. Review the WAI-ARIA authoring practices for checkbox.

Also, the element that signifies that the Publications menu has more options:

<i class="angle right icon"></i>

is not actionable and not associated with the checkbox. Also, the sub-filter elements cannot be selected unless the Publications element is selected. Consider removing the "show/hide" design of this element so that all options are exposed.