More WIP on controller specs for indexes
Update ExternalActivitiesController spec for Solr

The index action used search, so it needed to be rebuilt, and it was untested, so it needed tests.
ActivitiesController and ExternalActivitiesController specs

These are working specs for the controllers. They use a doubled Search model, so the returned results don't match actual search results; the primary goal is testing that the test double is called with the desired parameters.
rm incorrect spec - these behave as Activities
InvestigationsController#index works and is specced