Consider making `content` the config root

The setup of HM Base currently considers the root directory as the custom code root directory. As a consequence, PHPCS, PHP unit tests, the Composer file etc. are place inside of the root directory.

I do think however the it would be best the place most if not all of these things inside of `content`.

Because once you add NPM on top, you end up in weird scenarios where you sniff and lint from `content`, but run the PHP unit tests from a config file in root. If you have custom unit testing code, it's probably in `content`, and it all is rather confusing.