Mail service tutorial update

[piemonkey]

I had issues trying to get the mail service tutorial working inside mu-delta-service, so I realised it made more sense to base this update on the feature/add-auth branch, so I could use delta-notifier without having to write docs on how to set up mu-authorization. Right now this PR is very big, but that's because it's based on my tutorials branch, with the feature/add-auth branch merged in. If #12 is merged in and master merged into this branch, then those changes should disappear. If feature/add-auth is merged first, then I can rebase this PR onto the new master.

This fixes a few issues with the tutorial and makes it easily testable out of the box with a free email service (which I think was otherwise preventing people from actually using the tutorial code). I also added notes on how to test while working, to make the tutorial easier to follow along with and get working.