Create a hierarchical facet for name and role

[caaster]

Story: Exhibit curators would like to be able to enable users to facet first by a name and then by the roles associated with that name, to provide more granular discovery without forcing the user to click through multiple facets to find all objects associated with a particular name regardless of role. (SearchWorks supports hierarchical facets, but does not implement them for these specific data elements.)

Acceptance criteria: A hierarchical facet should be available containing all values of /mods/name/namePart, with a second level enabling faceting by each role associated with a name.

Fixture objects are in #889, and 1-5 apply

[cbeer]

This ticket is labeled "SearchWorks does it", but from the description it sounds like searchworks doesn't do this exactly. Maybe design is needed?

[arcadiafalcone]

The tag was meant to indicate SearchWorks already does hierarchical facets, though not for this particular field. Taking a look at it from the design perspective would be good.