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

[Stian Thorgersen]

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.