Internal notes

https://assemblymade.com/helpful/wips/158

This allows a user to mark a message as "internal" so it's only
available to agents.

Added here are:

* The migration for the 'internal' column
* A slider switch for choosing whom the message goes to on the form
* Proper user selection for e-mails based on 'internal'
* Display internal messages with a highlighted background.

No designs or styling for these elements has been conceived or presented
yet, so I used the most basic functional styles possible.

I'll note that I did NOT wrap whether or not customers can set the internal state.  Customers currently can't have user accounts at all (although they will some day in the future I'm sure).  When that happens, the whole app will need a once-over with proper permission limiting in lots of places like this.  Leaving this work until later ensures that the permissions are enforced consistantly.