Pass the current form instance to the callback as second argument

refs #5525
Move Icinga\Form\Config\ConfirmRemovalForm to Icinga\Form

This is a generic form not necessarily being used just for configuration
purposes.

refs #5525
Adjust Icinga\Form\Config\General* to use handleRequest() & Co.

refs #5525
monitoring/commands: Remove `CommandForm' which will be replaced

Remove the rubbish.

refs #6593
monitoring/commands: Rename `InstanceCommandForm' to `CommandForm'

Since the old `CommandForm' is removed, `InstanceCommandForm' is now the new base class for all command forms.

refs #6593
monitoring/commands: Let `ToggleFeatureCommandForm' extend `CommandForm'

`InstanceCommandForm' was renamed to `CommandForm'.

efs #659
lib: Remove `ElementWrapper' form element decorator

Zend has a `Label' decorator already.

refs #5525
lib: Remove `HelpText' form element decorator

Zend already has a `Description' decorator.

refs #5525
lib/form: Fix default element decorators but breaking noscript layout

Replaced `ElementWrapper' and `HelpText' with Zend's `HtmlTag' and `Label' respectively.

Instances of Zend_Form_Element returned by `Form::createElements()' now receive our desired default decorators.

refs #5525
Merge branch 'bugfix/rebuild-form-builder-5525' into bugfix/commands-6593