[#1687] Update webpack config to emit sourcemaps

[catarak]

Fixes #1687
I have verified that this pull request:

• has no linting errors (npm run lint)
• is from a uniquely-named feature branch and has been rebased on top of the latest develop branch. (If I was asked to make more changes, I have made sure to rebase onto develop then too)
• is descriptively named and links to an issue number, i.e. Fixes #123