fix for regression of file system date branch caused by multidump changes

the  old file system code used an insertion timestamp to build the date branch of the file system storage.  The new crash storage system does not send a time stamp with each storage insert.  The old code was then dropping back to get the time information from the crash_id.  However, the crash_id doesn't have hour and minute information.  The old code then just used 00Hr and 00min for everything.

The solution is for the old code to grab the 'submitted_timestamp'  (date_processed) from the crash itself.  That'll give the file system proper HH and MM so the distribution will be better.  

The change is embodied in socorro/external/filesystem/json_dump_storage.py:100 - the other changes are supporting code and unit tests.