Upgrade ui/build.sh
Use a separate purgecss.config.js configuration file, and drop the 'gofmt' step from the generation of assets.go: the file is already well-formed, and we no longer need a Go build environment when building the assets with npm.
Use a separate purgecss.config.js configuration file, and drop the 'gofmt' step from the generation of assets.go: the file is already well-formed, and we no longer need a Go build environment when building the assets with npm.