Email channel granularity (digest vs immediate send)

Different options in the `Send Message` sequence step to `Send Email` and `Add to Email Digest`. The email digest could be configured in user settings to send daily or weekly, while anything marked `Send Email` would be sent right away.

The digest would allow the user to keep tabs on their bot at a higher level without having to login to the app all the time. The immediate emails would be for more urgent/important matters that you wouldn't want delayed or mixed in with a bunch of other messages.