Add button to collapse all pull request files diffs #4333

Problem

When reviewing a fairly large PR the UI can get slow, especially if there's a lot of auto-generated stuff (lots of files, large snapshots, lockfile updates, etc).

Even though Github doesn't load large diffs by default, if you got tons of files with small diffs, it still causes slowness and creates literally an "infinite scroll" when you want to verify for something in specific.

Example PR: https://github.com/babel/babel/pull/5412/files

This one is not that bad, the one that made me thought about it is actually from a private repo that I can't share, but it touched 189 files (mostly editing 3 LOCs everywhere), and huge lock files updates (diff stat was +35000, -32000).

Suggested implementation

1. Button could be at the left of the "x / y files viewed" section. Could be a hotkey as well on the first file, i.e if you click it holding alt, it collapses everything.
2. Clicking would trigger something along those lines:

   document.querySelectorAll('.file-info > button:first-child[aria-expanded="true"]').forEach(node => node.click());