Avoid using Promise.prototype.finally to better support older devices

[peaBerberian]

Starting with the for-now-unreleased v3.30.0 release, due to the fact that we completely removed RxJS from the code, we started relying much more on Promises and on its related methods.

After encountering a curious issue on Panasonic TVs while investigating #1219, we saw that the finally method on Promises was poorly supported on some devices, and to our surprise even if a Promise polyfill was added.

After evaluating what we could do, I decided to just ban the finally method from the RxPlayer code and replace it by duplicating its logic in Promises' resolve and failure handlers.
To ensure that those methods are not used on future developments, I also added an eslint rule (and plugin) to ban the finally method. Sadly, this plugin does not check if a finally method call is actually done on a Promise instance, so I had here to just ban the method altogether (to prevent future confusion, I clearly indicated in the error message that the linting rule was naively banning all finally method calls).

I also tried to override a Promise's finally method through TypeScript, but this solution appeared much more hacky than I first thought. I think a linting rule is a better solution for now.