The email preview iframe height is fixed and does not scale to the browser size

On larger screens the email preview iframe is being limited to a height of 800 pixels, and the full available screen size is not being used.

I've just updated the height of the iframe and parents to be explicitly 100% so that the full height of the browser window can be used.

I've also created a pull request for the standalone mail_view gem https://github.com/basecamp/mail_view/pull/80