add sitegroup handle & more defined template paths

[ThomasDeMarez]

Description

Simple implementation of Site specific templates inside of a specific SiteGroup. Uses auto generated SiteGroup handles based on the group name.

.
└── templates/
    ├── SiteGroupHandle/
    │   ├── SiteHandle/
    │   │   └── index.twig ----> specific template for site with this handle in this group
    │   └── index.twig     ----> default template for all sites in this group
    └── index.twig.        ----> fallback for all other sites

Related issues

#3101