gluon-luci-core: replacing luci-mod-admin-core

This commit adds gluon-luci-core, a package both gluon-config-mode and
gluon-luci-admin will depend on. gluon-luci-core itself will depends on
gluon-luci-theme, gluon-core and most importantly luci-lib-web. It also
copies a few files from luci-mod-admin-core but leaves out everyting we
do not use.

At a later point I plan to move functionality from luci-lib-web into
this package, too. luci-lib-web contains most parts of luci including
hardcoded templates for CBI, which I intend to replace.