1758: Ensure all external links have the same values for the rel attribute

[stevejalim]

For links that use target="_blank" we already had rel="noopener" set and sometimes nofollow and sometimes noreferrer, but this changeset brings them all into line so that ALL of them use nofollow noreferrer noopener for consistency and predictability. (I've also seen this pattern used by Mozilla Hacks blog.)

This does raise questions about SEO and analytics, regarding nofollow and noreferer and we could in the future support fine-grained control over those values for links.

Please feel free to disagree!

(Related issue #1758)

How to test

• code will be on staging soon, so look at external links or social sharing links. However, what's more important is consensus on whether nofollow is definitely worth it and whether we are comfortable with/without noreferrer

[stevejalim]

@valgrimm May I grab a view on this from you as PO?

[valgrimm]

Need Chris' take on this on Thursday, deferring to his judgement as discussed

[valgrimm]

Chris and I talked today about this and he said that it is fine to leave off the noreferrer header so other Moz properties know we are sending them traffic.

[stevejalim]

For now, in the circumstances, am merging this as-is, and the rel attr can be loosened off in the future, if need be