WIP: Migrate to Webpacker
What does this MR do?
Migrate to use Webpacker gem instead of our custom code.
Should work and behave similarly to today. By introducing Webpacker, we can also opt to use with webpack-dev-server or with the on-demand compilation (useful when not developing frontend, as you will not need to keep webpack-dev-server loading on the background.
By moving to Webpacker and using its own defaults, we also reduce the need to explain to contributors how we are integrating rails with webpacker and the conventions we established (we will use Webpacker conventions, so people don't need to learn our implementation details in order to be able to contribute).