Show accurate experience notifications (#10676)

* Show accurate experience notifications

Add unit tests for exp notifications

* use array to compute exp and lvl values for notification changes

* Add tests for user loosing xp cases

website/client/libs/notifications.js

@@ -47,6 +47,5 @@ export function round (number, nDigits) {
 }
 
 export function getXPMessage (val) {
-  if (val < -50) return; // don't show when they level up (resetting their exp)
   return `${getSign(val)} ${round(val)}`;
 }