[Course Search] Allow redirect to the unit page from the search results

[bradenmacdonald]

From #200

If the user clicks on a result that is a component from a unit, we need to be able to redirect to that unit.

We need to add to the component document a "parent" usage_key that contains the unit key.

It can either be a new field or the existing "breadcrumbs" field already has a place for it - currently the parent breadcrumb only has "display_name" attribute but we could easily add "usage_key" as another attribute on the parent breadcrumb too.