Add Twitter Card Support in Redirector

[jamesward]

This will look nicer when deploy.cloud.run urls are used in tweets.

[glasnt]

I believe this would require changing the cmd/redirector:redirect failures to return rich output, including the appropriate opengraph/twitter tags/images.

As for what the image shows, there is a social preview available for this repo itself (though the button shown is out of date(?))).