Feature/user stats

- Includes @jonlemmon's change to the stats export so that it processes the users in batches. 
- Adds 'last_sign_in_at' to users export so we can filter stats by active users. 
- Looks for an environment variable 'LOOMIO_INSTANCE' to differentiate on S3 between stats csvs generated by different copies of Loomio
- Adds rake task to export (without scrambling for privacy) all events associated with a specified group. It's a first cut, and quite ugly at the moment, in that it takes just as long as the task to export all events.