Multi-select facets should show checkboxes

[kmthorn]

Descriptive summary

While a user is searching it is not obvious that they can select multi-facet values. Other places have gotten around this through checkboxes. This seems to be the web standard and we should do the same.

Expected behavior

Checkboxes are visible and selectable for multi-facet value selects

Accessibility Concerns

From QA: #1409

Be careful with checkboxes.