Configuration can be created with an unknown langcode

Problem/Motivation

This is a follow-up from #3337864: Ensure "Site default language' is used when installing config by any means and is updated correctly which fixes the case where a module install creates configuration in english only to have locale try and fix it later.

Steps to reproduce

Add configuration to a module with a langcode not supported by the site - install the module. The configuration will exist in this language.

Create a configuration entity with an unsupported langcode or a simple configuration object and save it.

Proposed resolution

Change the langcode as part of the ConfigInstaller and either:

• skip it
• change it to the default langcode
• error about it using \Drupal\Core\Config\ConfigInstaller::checkConfigurationToInstall()

It's trickier to know what to do about runtime (as opposed to install time) checks - potentially we should just recommend using config validation as that will error on this problem already.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet