Fix underscore dependency (moved underscore-min.js to /client/app/lib)

I had underscore in /client/vendor locally.  /client/vendor is in .gitignore and its contents are managed by bower, but underscore was not installed via bower - I just copied the file manually. It was working fine locally, but failing on heroku since it never got installed.

From now on, manually-installed dependencies will go into /client/app/lib to prevent similar problems in the future.