[Backport to 11.1] Webpack based frontend-build

The context-path of the CMS can be anything: /cms, /cool-cms or /. For this reason, the sources built with the frontend-build should be required with a relative path instead of an absolute-path. For example, currently the angular resources of the new channel manager are hardwired to /cms/angular/hippo-cm.

The path can be made relative by changing the frontend-build configuration, but this breaks the current development proxy servlet. For example, the webpack-dev-server of the channel-manager serves requests at http://localhost:9090/cms/angular/hippo-cm/** but when we change the frontend-build to use a relative path, it will serve files from http://localhost:9090/**