Re-work the button styling

[MelissaAutumn]

The existing button styling is a small mess due to me not wanting to refactor the download button. We should stick with a btn class that applies button styling to anything. As well we should have mixin classes for things like transparent or white filled backgrounds, applying hero button background image, etc...

We have some additional button states from https://github.com/thunderbird/thunderbird-website/pull/592