Add a route_set parameter to some of the link helpers.
Thie enable blacklight to be used by another Rails engine, without
having to reimplement a number of the helpers. See
https://github.com/projecthydra/sufia/commit/9b495ae76e55aef505cb070fff0b325a31ace356
Merge pull request #650 from projectblacklight/engine_scope_links

Add a route_set parameter to some of the link helpers.