Generate summary data for certificates

Opening PR for discussion. Fixes theodi/shared#56

This PR creates a few more Rake tasks to generate CSV for the stats requested in theodi/shared#56, as well as others shown in [/status](https://certificates.theodi.org/status). They include counts for:

* All surveys started
* All Certificates
* All Datasets
* Published Certificates
* Published Datasets (I *think* this covers the 'how many certificates created for new data' requirement in theodi/shared#56)
* Raw level Certificates
* Pilot level Certificates
* Standard level Certificates
* Expert level Certificates

There are two rake tasks, one that sets up the CSV file (should probably only be run once manually), and one that needs a daily cron job to add a row of stats for that day.

Thoughts are welcome!