front-end: Introduce 'front-end' directory with source files for building the front-end The top level with package.json is not included when installing with pip, and 'kallithea-cli front-end-build' and npm would thus fail. Instead, introduce 'kallithea/front-end/'. It is under 'kallithea/' and is thus included when doing pip install, but it is just a source directory and not under "public". Most of the "less" stuff should probably move there. And probably also most things that now are checked in under "public", so a fully populated public front-end directory can be built anywhere, without write access to the Python installation directory.