Stats export improvements

Groups export: adds distribution_metric
Users export: adds last_sign_in_at (empty if they never have)
Events export: 
- adds 'top_group', which is the top-level group associated with each event (i.e. the parent group if it's a subgroup)
- scrambles parent_group properly (correcting a little bug)

Save to S3: Now looks for an environment variable 'LOOMIO_INSTANCE' to differentiate on S3 between stats csvs generated by different copies of Loomio