[FEATURE] EXT:form - integrate new form framework

The main purpose of this patch is to integrate a flexible framework for
building forms. It replaces the legacy 'form wizard' based on ExtJS and
the depending frontend rendering system.

The new backend 'form editor' relies on vanilla JS and jQuery.
Different JS patterns have been applied to ensure a modern architecture,
high flexibility and extensibility.

A new backend module lists all existing forms and allows the creation
of new ones. The 'mailform' content element is reworked. It lists
available forms and enables the backend editor to override certain
settings, e.g. 'finisher' settings (formerly known as 'postProcessors').

Till now it was not possible to customize and extend the 'form editor'.
To allow the registration of new finishers, validators and
pre-defined form elements a lot of architectural changes were needed.
After a long conceptional phase the team decided to remove the former
code base, backport the 'form' package of the Flow project and ...