Build layer configuration web interface

[fergonco]

A tool where the user can log in, modify the layer tree and save the status would make the portal much more easy to configure. Many of the support request I get are about a broken layers.json and it's one of the hardest part to teach in trainings, so it would be a good investment.

There is an exercise in the Spanish developer training that shows a way to implement this allowing to change the center of the map:
Service:
https://github.com/nfms4redd/documentation/blob/develop/workshops/es/developer/ejercicios/server-modules/GuardaCentroServlet.java
Client modules:
https://github.com/nfms4redd/documentation/blob/develop/workshops/es/developer/ejercicios/client-modules/guardar-centro.js
https://github.com/nfms4redd/documentation/blob/develop/workshops/es/developer/ejercicios/client-modules/botonera.js