rewrite public file stat metrics
rewrite public file stat metrics

 * Since MFR caches some responses to WB, we can't always rely on
   WB-side metrics to give an accurate count of views.  Instead both
   services will be updated to log their respective actions to a common
   collection.  As part of this, the public event is now constructed by
   a function that can be shared between both services.  Previously we
   had been copying the private payload, then attempting to sanitize it.

TomBaxter/waterbutler,Johnetordoff/waterbutler,felliott/waterbutler,RCOSDP/waterbutler,rdhyee/waterbutler,CenterForOpenScience/waterbutler