Make it possible to provide a closure to be called instead of onSuccess()

This allows us to write generic forms like the ConfirmRemoval form but to
utilize handleRequest() while providing a closure in the array that is passed
to a form's constructor.

refs #5525
Icinga\App..\Config: Rename __construct to fromIni and add setConfigPath

Prior to this change it was not possible to create an instance of Config
without passing a config file.
Add base form class for configuration forms

refs #5525
Adjust authentication backend forms to suit.. some form implementation

refs #5525