[CAL-3685] Introduce class WebAppURL that extends URL one

[zomars]

Relying on string concatenation to for URLs is prone to a lot of typos and confusion. We can use URL class for this.

Also we can extend it so it has the WEBAPP_URL as base by default.

Not a blocker but let's rely on constructing URLs using the URL class or our own extended one.

https://nodejs.org/api/url.html
https://developer.mozilla.org/en-US/docs/Web/API/URL/URL

_CAL-3685