As discussed with the product team we want to enforce kebab-case file names for
all files, with the exception of files which export a single class, in which
case they should be PascalCase and reflect the class which they export.
This will help find classes faster, and should push better naming for them too.
Some files and packages have been excluded from this linting, specifically when
a library or framework depends on the naming of a file for the functionality
e.g. Ember, knex-migrator, adapter-manager
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
