Two fixes:
* In the (in-repo, non-Tutor) Dockerfile, add copy-node-modules.sh
before `npm install`, since it is needed by the new postinstall hook.
* In paver/assets.py, run copy-node-modules.sh for backwards com-
patibility, just for cases where `SKIP_NPM_INSTALL` is enabled
(which would prevent our new postinstall hook from running
automatically!). We will deprecate the paver asset commands all at
once once the new non-paver stuff is 100% working.