Allow choosing of parent templates for static views

Requires tackling several issues:

• all templates in certain folder a considered "parent templates"
• optionally validate parent templates (have to contain block directives)
• extract all possible block elements of a selected parent template indicate it's name and provide a form element for each block
• revisions must store a picked template
• solve potential problems arising when changing between templates with different blocks (delete content which can not be assigned to blocks might be a simple and robust solution)