Avoid duplicate content caused by `view-markdown-source`
Avoid duplicate content caused by `view-markdown-source`

This now disables the button while loading like GitHub does when clicking "Show rich diffs" in SVGs

Fixes https://github.com/sindresorhus/refined-github/issues/1935#issuecomment-484849946

sindresorhus/refined-github,busches/refined-github,sindresorhus/refined-github,busches/refined-github