Explicit project subject count

This change just makes Project.subject_count an explicit variable so is mainly for readability (it would bind anyway if sent from API call)    However, only accept this if a back-end change to Ouroboros to set subject_count also goes through.  

Currently, this field is rarely sent from API call /projects/$PROJECT_NAME.  However, Panoptes will most likely send it.   After some discussion with @edpaget, we decided this was probably a reasonable de-normalization to have on Ouroboros.  Projects that needed to have comprehensive stats on the front end would otherwise have to make multiple fetches on Groups.