Refactor Activity api to more efficient query

The activity api is doing a per-row permission check. We should refactor to be a tonne cleverer following on from CRM-20441

I think the first step is to remove VIEW calls to here:

CRM_Activity_BAO_Activity::checkPermission($this->_activityId, CRM_Core_Action::(VIEW);

Then only the api will be using that for view-based checking & we can clean up the way it checks related tables, possibly separating view from edit.

We should also look at the retrieve function on the activity BAO with a view to removal & using the api for similar reasons