If you open a file from a PR that contains an unsupported file type, it automatically opens your browser and navigates to the GitHub website and shows the preview. This is quite jarring and we should provide a better experience by showing a message and giving the user an option to preview on the web.

Repro steps

1. Navigate to PR that has changes to an svg/ttf/unsupported file type (i.e. Add .glsl icon jesseweed/seti-ui#504)
2. Click on file in the GitHub PR Viewlet
3. You are re-directed to the GitHub that shows the file

Expected

1. Show a message that explains that file is not supported with a link to preview on the web