Add filter to show events and materials with broken links

[fbacall]

Currently, events and materials with broken links are hidden using a solr facet that is always applied (unless you are an admin):

TeSS/app/models/concerns/searchable.rb

Line 111 in b36c528

without(:failing, true)

Replace this with a boolean facet in the sidebar, as we do for include_expired and include_hidden.