[keycloak-dev] Remove built-in themes from themes directory

See thread "Shared messages between themes". I propose to remove built-in themes from themes directory and rather load from a module/jar. Reasoning behind this is two fold: a) Support theme resources for sharing messages between themes b) Built-in themes should not be modified Within themes we'd update the README.txt file to provide the location of the built-in themes so users can crack open the jar (zip) to view the built-in themes for reference when they are creating their own custom themes.