refs https://github.com/TryGhost/Toolbox/issues/363
- this commit pulls all code involving the Mailgun client SDK into one
new package called `mailgun-client`
- this means we should be able to replace `mailgun-js` (deprecated) with
`mailgun.js` (the new, official one) without editing code all over the
place
- this also lays some groundwork for better testing of smaller
components
