tools/reviewer/webapp should build and run with Bazel instead of npm #353

It's in the monorepo which uses Bazel for the build, so the frontend should be built with Bazel too!
The build process right now is a manual npm run build after the bazel build which doesn't scale well once more projects are introduced and all these npm run build steps have to be run separately. It should take advantage of the speed of Bazel!

Here is an example Angular project building with Bazel: https://github.com/alexeagle/angular-bazel-example