[TASK] Add optional categories selection

You can enable the category selectbox with
typoscript.

There are 3 options for 'settings.categories.enable':

# empty = disabled
# 1 = multiple categories selection
# single = single category selection
[TASK] Add typoscript switches for non required fields

It is now posible to disable the non required fields
by typoscript
[TASK] Fixed missing translations

And added Dutch translations
[BUGFIX] recipientMail check in controller fails

The recipientMail check in the controller fails when
the setting isn't present.

Made the check less strict so it can handle FALSE, NULL
and empty strings.
[CLEANUP] CSS cleanup

Moved all css to setup.txt
[CLEANUP] Remove default Error FlashMessage
[CLEANUP] General cleanup

Removed unnecessary files and configuration.