Make logback.xml a build time configuration resource

Currently logback.xml is located in the platform-web module with a default, xwiki-specific, configuration.

Anyone building an XWiki-based distribution can not modify that configuration and the only option is to completely overwrite the logback.xml file in the output war of the new distribution.

To improve this, logback.xml should be configurable at build time and part of the xwiki-platform-tool-configuration-resources module so that the war plugin filters it and adds any extra configuration needed by the new distribution.