Fix Mass Import Memory Exhaustion

.. don't signal when adding a ride, which kicks off
   all manner of updates across the code, instead
   we let the metric refresh kick in at the end.

.. this is great for an initial load and mass import
   of large numbers of rides, but sucks for a small number
   of rides when maybe downloading

.. we could make the signalling dependent on the number of
   rides being imported, so small number still updates the
   CP charts etc.

THIS IS A TEMPORARY FIX TO THE MORE SIGNIFICANT PERFORMANCE
ISSUE -- BUT WE DO NEED TO CONSIDER THE ISSUE OF CHART UPDATES