[Backport to 11.1] - Reintegrate Webpack build

The Webpack-based build used in the feature/visual-editing-psp1 branch makes development much faster and pleasant. We like to reintegrate it into master before the actual refactoring.

After reintegrating to master, merge master into feature/visual-editing-psp1 and resolve all merge conflicts while memory is fresh. The reintegration of feature/visual-editing-psp1 into master will then be painless (at least w.r.t. Webpack).