Backport (HSTCONFIGEDIT-115): Enable freemarker editing from within hst-configuration-editor

Enable editing of the freemarker templates within the Hst Configuration editor environment.
Validation needs adjustments, the field 'renderpath' is not longer a required one if one has filled in an ftl template.
For now a textarea should suffice for pasting in the ftl template. Future plan is to integrate with some code editor (a javascript one, like code mirror.