[FEATURE] Introduce Site Handling

TYPO3 is famously known for the "multi-site"
functionality, allowing multiple websites running
within one TYPO3 instance.

However, configuring a multi-site had various downsides,
mostly regarding to domain/entrypoint handling for a site,
and if lots of languages were in place.

Concepts like "absRefPrefix", "baseURL", various language
related TypoScript settings, and the infamous
"L" GET parameter can now be seen obsolete.

Also, handling page-not-found or access-denied errors
have never been easier, as every admin/integrator is able
to configure this.

What TYPO3 calls a "site" is a entrypoint / pagetree,
and contains both configuration values relevant for
Backend and Frontend.

A site configuration has a unique (human-readable)
"site identifier" and the following additional values:

* Root page ID
This is a page on the root level (pid=0) or having
"is_siteroot" checked.

* The base path / base URL
This HTTP entry point e.g. https://www.mydomain.com/
 ("Base URL" / HTTP...