Replace bower-rails with webpacker

[Xavier Redó]

[Pablo Rivera Conde]

Note that Rails 5.1.0 does have support for webpack. As the changelog say:

Add Webpack support in new apps via the --webpack option, which will delegate to the rails/webpacker gem.

To generate a new app that has Webpack dependencies configured and binstubs for webpack and webpack-watcher:

rails new myapp --webpack

To generate a new app that has Webpack + React configured and an example intalled:

rails new myapp --webpack=react

DHH

Source: https://github.com/rails/rails/blob/v5.1.4/railties/CHANGELOG.md