Adds a Site::Layout module that handles site_layouts.yml
Ensure that at least the default Site is returned for `Site.current`
[FEATURE] Adds Site Layouts

Site layouts are similar to page layouts. You can define site layouts in `config/alchemy/site_layouts.yml`.

For now you can define page layouts that are only available for a specific site.

== Example:
  # config/alchemy/site_layouts.yml
  - name: default_site
    page_layouts: [contact, imprint]