Add rel=preconnect link for AMP CDN

[westonruter]

In a support topic I found out that Lighthouse audits are flagging the lack of rel=preconnect links for https://cdn.ampproject.org:

This currently is not part of the AMP optimization guide, and I thought it was unnecessary because there is already more-specific preload links, like:

<link rel="preload" as="script" href="https://cdn.ampproject.org/v0.js">

I checked with @sebastianbenz and he pointed out that not all browsers support rel=preload, so adding a rel=preconnect link would be a good thing to do.

[westonruter]

Only room for improvement here is that I'd like it if the page already had this link that it would be prevented from being duplicated.

[schlessera]

That would then come from a third-party plugin/theme instead? It would be difficult to detect, it might be easier to sanitize the duplicate away instead.

[westonruter]

Potentially. We actually can detect because we gather up a list of all the links right here. So we could just skip adding it if it already exists.

Aside: There is also a need to remove component scripts that do not exist among $script_handles, to remove scripts that are not relevant to the page (in case someone manually adds a script which turns out to not be relevant to the current page). See #3489.

[westonruter]

In practice there should not be duplicates. After thinking about it more, it seems wasteful to detect for a duplicate link[rel=preconnect] for AMP, and it doesn't hurt to have it duplicated in the first place.

[amedina]

LGTM. Ship it.