replace bower with npm and Flask-Assets with webpack

The npm configuration:
- Includes linting of javascript with eslint

The webpack setup here:
- Bundles javascript and css, using hashing to support server-side
  caching, with support from Flask-Webpack and
  manifest-revision-webpack-plugin
- Uses babel to transpile ES6 javascript
- Includes a development server with hot-replacing of javascript and
  css code