Making PR.s easier to diff

1. Accordion style file headers

   • When diffing a PR, the file headers work like an accordion now!
   • Click once to open, click again to close. And opening one doesn't close the others.
   • There is also a "collapse" button added to the page.

2. Ignore whitespace button

   • When diffing a PR, a new button is added that will toggle the whitespace, like the git command git diff -b

This pairs nicely with github-wide userstyle https://github.com/mdo/github-wide