PSR-12 compliancy

* Replace mock-test with actual test

Add support for RFC6265bis SameSite cookie attribute

This reverts commit 6bcea51e.

Stream wrappers were assumed to be relative paths. When using a stream wrapper (e.g. s3://) it was appended to the given base path.

The old mail class uses the built-in `mail` function in PHP, and builds a payload by itself using hard-coded HTML and MIME-snippets. Let's use a library for that; PHPMailer has new dependencies and does all the heavy lifting.

Also make the default namespace parameter mandatory, so that the function is not ADFS-specific.