Add a global file hierarchy level

[emirror-de]

Motivation
Currently only two file hierarchy level exist, modules and pages. If resources need to be shared throughout a whole website, it would be required to either symlink or copy them from another page level. This creates massive overhead in disk space and a significant amount of work for the developer.

Desired solution
Introducing a global file hierarchy level that is automatically shared throughout all pages.

Code examples/public API

pub enum Level {
    Core,
    Module,
    Page,
    Global, // maybe another name would be more appropriate?
}