Modernize front end

[anarute]

The front end of the project is based on a very outdated version of ExtJs.
If I understood correctly the project is not open source anymore, so I would suggest migrating to a modern open source framework/library, such as React, Vue or Svelte or even vanilla js.

This is a big change so it could be done in chunks.

[anarute]

Btw I'm tempted to go with Vue, I was sold with:

Unlike other monolithic frameworks, Vue is designed from the ground up to be incrementally adoptable. The core library is focused on the view layer only, and is easy to pick up and integrate with other libraries or existing projects

From https://vuejs.org/v2/guide/#What-is-Vue-js
I worked with it before and it was pleasant, but I'm not an expert.

I'll give it a go in the holiday management feature as a trial.

[anarute]

First experiment adding Vue.js was done with #512 and I think we should use it across the whole project: fast learning curve, easy to setup and can be done incrementally.