Add the possiblity to load custom js on startup

[freddidierRTE]

Add in web-ui an array of string defining js file with relative url to be loaded on startup

param name proposal : "customJsToLoad"

The base path for js should be /externalJs

Add a fictive example that just write in the console "Custom js example executed" in a file name example.js in external/

See how we can be sure the script has been loaded before loading a template , using await in the loading sequence should be possible