This keeps the old "react-app" directory in its existing location (to make
it easier to merge changes from the main branch), but separates it from the
npm workspaces setup. Thus it now needs to be npm-installed/built/linted
separately. This is a bit hacky, but should only be needed temporarily,
until the old UI can be removed.
Signed-off-by: Julius Volz <julius.volz@gmail.com>