mark user as inactive on unsubscribe
minor refactor
update experiment completed summary email

This is a point of dirtiness, we shouldn't need to update this email
every time we run an experiment. Consider templating payment details
via an extra Experiment field.
update criteria with other filter kwargs
remove defunct robots.txt paths
fix NoneType error in email validation

Only happens for non-HTML5 compliant browsers with JS turned off?
sort all_activities by date created