Add (temporary) SNS notifications of account delete events.

As discussed in #608 and https://bugzilla.mozilla.org/show_bug.cgi?id=971907 we'd like to send notification of account deletion events via SNS, so that downstream apps can cleanup any data they have stored for a user. 

The ultimate plan is to have this managed by heka, but heka doesn't currently have SNS support and I don't want to block on fiddling around with that.  I think we should consider having the app do it in the first instance, so we can work on the downstream flows and the heka pieces in parallel.  This is my WIP attempt at some minimal temporary code in the app to manage this.

Not ready for merge, but @dannycoates f? on this as a temporary measure until we get the heka pieces in place?