Move Heading/Footer to shared blocks

[remkoj]

In the current code base the Header & Footer are part of a special Page Type "StartPage", this creates the implicit rule that all applications must have an item of this type as Root for the front-end to work.

This must be changed to shared blocks, which can be either linked to a specific application or be used as a generic fall-back so it's not needed to manually change the content to link the Header and Footer to an application if there's only one.

Required:

• Layout block type that allows configuration of the header & footer
• Page "Export Settings" with reference to these blocks within the site, published, but only readable by CmsEditors & CmsAdmins
• Updated setup documentation in README.md