dillon1000/react

[Danger] Minor fixes (#12606)

* Don't download bundle stats from master on CI

This was temporarily necessary in the past because we didn't have the logic that downloads actual *merge base* stats.

We do have that now as part of the Danger script. So we can remove this.

* Use absolute threshold for whether to show a change

* Download master stats, but only for other master builds

* Rewrite sizes