- the tests here were no longer relevant because they were more testing
things that have been moved to `mailgun-client`
- this commit cleans up the tests to ensure we're passing the correct
parameters to the mailgun client package
- also adds jsdoc on all the functions and maintains 100% code coverage
- this test checks that the mailgun client respects the changes in
settings, which is something that we used to ask
`email-analytics-provider-mailgun` to do when the mailgun client was
made in that package
- since then, we've pulled it out, so we should move the test to the
`mailgun-client` library
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
closes: https://github.com/TryGhost/Ghost/issues/14640
- eu mailgun domains have a different structure.
- we weren't accounting for this when fetching the next page of results, meaning that email stats didn't work on EU domains
