Set ignore_unknown when inserting to BQ

This means that if a schema in BQ lacks a field in our schema, it won’t
cause an exception and the (presumably blank) field will just be dropped

lib/dfe/analytics/send_events.rb

@@ -17,7 +17,7 @@ def perform(events)
         if DfE::Analytics.log_only?
           Rails.logger.info('DfE::Analytics: ' + events.inspect)
         else
-          DfE::Analytics.events_client.insert(events)
+          DfE::Analytics.events_client.insert(events, ignore_unknown: true)
         end
       end
     end