RTL από Thibaut LaBarre

Adds an RTL button on GitHub PRs. The button will allow developers to easily copy a link to review the PR that can be posted in Slack. Also defaults the diff view to hiding whitespaces.

For example:
1. Go to: https://github.com/prowriting/beyondgrammar-quill/pull/31
2. Click on the RTL button
3. Paste the corresponding link: "RTL: Fix typo [+1/−1]"