Streamline Logging in a bunch of components

[schmitz-ilias]

The Logging component was recently refactored and cleaned up as described in Streamline Logging. This PR contains the changes required to properly conclude the implementation of that feature:

• The docs related to installation and configuration of ILIAS are updated to include the new default_level field in the ilias.ini.php. Also for the master.ini.php. (ping @thibsy, @chfsx, @acgruber)
• All components and plugins now automatically get a configurable log level, so the logging tag is removed from all service.xmls and module.xmls.
• The long deprecated ilLogger::write and ilLogger::writeLanguageLog have been removed, and its usages replaced (mostly with ilLogger::info). This makes the public interface of the now deprecated ilLogger match the new LoggerInterface.
• (Almost) all usages of the root logger have been replaced with the logger of the appropriate component. The exception is the logger exposed through an ancient global ilLog \ log. This is still used by a handful of components, most crucially Init\ErrorHandling, but tracking down all usages exactly is tricky. That logger identifies itself now as legacy_root in the log, and only uses the default log level.

Especially the last two sets of changes should be understood as transitional. In the long run, only the new LoggerInterface should be used, but the new infrastructure will only be directly available to other components when the Component revision is a bit further along (see #11430). Until then, this is a step in the right direction, and will ease the transition later on.