Email alert api support

Adds support for the new [Email Alert API](https://github.com/alphagov/email-alert-api)

I've tried to follow the patterns of the other tests where there are a some of helpers that set up Webmock stubs however this has led to a HUGE amount of test code for a trivial implementation.

Suggestions?
I prefer using `context` here -- the wording you've used for this describe statement is the context in which the text (`it`) runs, so stating that makes the test parse better in my brain.