Adds back an email field to be used as the sender address

[abea]

The email field was removed to allow sending results to multiple email addresses, but it's still needed to send messages from the app in some transports.

The test change was because there was no longer a namespace property on the collection, though a collection did come back. I used the core Apostrophe attachments.js test as a guide for that change.

[boutell]

apostrophecms/emulate-mongo-2-driver#2