Issuer is customizable in configuration so that a company can set its own name
or website. If not provided, default value is 'authelia.com'.
The username is used as label.
This refactoring aims to ease testability and clean up a lot of soft touchy
typings in test code.
This is the first step of this refactoring introducing the concept and
implementing missing interfaces and stubs. At the end of the day,
ServerVariablesHandler should completely disappear and every variable should
be injected in the endpoint handler builder itself.
Clement Michaud