Add a ThemeConfig service for setting and getting default and admin themes

Problem/Motivation

ThemeHandler has a setDefault method which really is not appropriate. See #2416673-41: Add a ThemeConfig service for setting and getting default and admin themes → .

Proposed resolution

• Add a ThemeConfig service that extends a (new) ThemeConfigInterface.
• Add methods: setDefault(), getDefault(), setAdmin() and getAdmin().
• Mark ThemeHandler::setDefault() as deprecated.

Remaining tasks

• Write a change record.
• Open a follow-up issue to replace usages of config->set(), config->get() for getting and setting default and admin themes.
• Open a follow-up issue to replace usages of ThemeHandler->setDefault() and ThemeHandler->getDefault()

User interface changes

API changes

Data model changes

Problem/Motivation