chore(ui): migrate from yarn to npm

nodejs 10.16.0 includes npm 6.9.0 which fixes previous issues with npm in LTS nodejs (lock diff on different platforms) so we can move back to using npm instead of yarn. The main problem with yarn is that it's harder to manage the version used in different places (CI, docker, local dev) while npm is bundled with nodejs so doesn't suffer from the same problem

.travis.yml

@@ -12,7 +12,8 @@ defaults_js: &DEFAULTS_JS
   # we run make to install everything
   install: []
   cache:
-    yarn: true
+    directories:
+      - ui/node_modules
 
 jobs:
   include:

Makefile

@@ -33,9 +33,9 @@ endif
 	GO111MODULE=on go install github.com/golangci/golangci-lint/cmd/golangci-lint
 	touch $@
 
-.build/deps-build-node.ok: ui/package.json ui/yarn.lock
+.build/deps-build-node.ok: ui/package.json ui/package-lock.json
 	@mkdir -p .build
-	cd ui && yarn install
+	cd ui && npm install
 	touch $@
 
 .build/artifacts-bindata_assetfs.%:
@@ -45,7 +45,7 @@ endif
 
 .build/artifacts-ui.ok: .build/deps-build-node.ok $(ASSET_SOURCES)
 	@mkdir -p .build
-	cd ui && yarn build
+	cd ui && npm run build
 	touch $@
 
 bindata_assetfs.go: .build/deps-build-go.ok .build/artifacts-bindata_assetfs.$(GO_BINDATA_MODE) .build/artifacts-ui.ok
@@ -126,7 +126,7 @@ test-go:
 
 .PHONY: test-js
 test-js: .build/deps-build-node.ok
-	cd ui && CI=true yarn test --coverage
+	cd ui && CI=true npm test -- --coverage
 
 .PHONY: test
 test: lint test-go test-js