Refactor assets to ES modules

[jiripudil]

It seems that more and more and more people are using ES modules and tools such as webpack to bundle client-side code, and datagrid's assets are not ready for such approach.

I'm opening this issue to keep track of the progress on this.

[dakorpar]

There were few tries before and few issues regarding this here... Everyone wants it. But no one ever prepared any PR. If U do it I'm sure they will gladly accept it, most of contributors to datagrid are mostly backend devs also so it's a bit trickier for them...