Don't clear old dotstorm drawing revisions on edit

This preserves their appearance for activity history view.  They will
still be removed if the idea is deleted (though deletion isn't there
yet).
Activity summary rendering and sending

Finish implementing the full rendering and sending pathway for daily
activity emails, to be fired by a cron job executing
`bin/send_daily_activity_summaries.js`.
Don't escape html in plain-text email.