Feature: #85164 - Available languages respects site configuration settings

See Issue #85164

Description

When the backend shows the list of available languages - for instance in the page module language selector, when editing records and in the list module - the list of languages is now restricted to those defined by the site module.

If there are for instance five language records in the system, but a site configures only three of them for a page tree, only those three are considered when rendering language drop downs.

In case no site configuration has been created for a tree, all language records are shown. In this case the Page TSconfig options mod.SHARED.defaultLanguageFlag, mod.SHARED.defaultLanguageLabel and mod.SHARED.disableLanguages settings are also considered - those are obsolete if a site configuration exists.