Use CloudMailin for incoming emails

[bsiegert]

• [ ] write handler that speaks the cloudmailin format
• [ ] unsubscribe App Engine inbound mail service
• [ ] subscribe cloudmailin
• [ ] remove App Engine inbound mail handler

[bsiegert]

Not going to do this. Instead, the standalone version can use something local. In my test instance, I use fetchmail with curl as MDA.