refs https://github.com/TryGhost/Team/issues/3337
Moved current email customization functionality that is behind the
`makeItRain` to its own flag (`emailCustomization`) and removed the now
redundant `makeItRain` flag
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
