The Webpack config for the modern build was adjusted recently to add a loader,
which is used as a workaround for a bug introduced by a recent dependency
update. The legacy config needed the same update.

The Webpack config was also updated in superficial ways to follow
recommendations made for migrating from Webpack v1 to v2 (e.g. using
`module.rules` instead of `module.loaders`). These changes should have no
functional impact, and should make migrating to future versions of Webpack
easier.

fix #6269