RTL incompatible value

[yarons]

https://github.com/letsencrypt/website/blob/9433cb2138482b4be62012ae278b6651a19c4791/src/css/_base.scss#L72

This property is strictly for LTR languages, for RTL we will need to allow an RTL variant of it (margin-right).

I'm not sure how to add the correct variant but we've added html[dir=rtl] in the past.

Thanks.