Add npm lockfile to node services

[bcongdon]

We should add a npm 5.x lockfile to all the node services.

Services that need lockfiles:

• [x] https://github.com/acm-uiuc/groot-desktop-frontend
• [x] https://github.com/acm-uiuc/groot-events-service
• [x] https://github.com/acm-uiuc/groot-groups-service

Required work: For each service: Using npm 5.x, run npm i in the service's root directory. Commit the package-lock.json file to the repo.

[narendasan]

Should we move to Yarn?

[bcongdon]

I don't think there's an especially compelling reason to switch to yarn at this point.