Notifications: SMS support

[doshitan]

Context

We had originally set up "notifications" to go through Pinpoint in part to more seamlessly support SMS in the future, but Pinpoint is no longer supported: #942

Implementation

SMS support on AWS is through AWS End User Messaging (related Terraform resources). Likely needs at least a env-config/notifications.tf setting for opting into SMS support, possibly a separate top-level enable_ option.

We could also consider supporting third-party SMS services in at least setting up the config interfaces, but the AWS service is the initial backend target implementation.

Success criteria

Services with enable_notifications = true can send messages to a phone number.