[Web vitals] Homepage and post page

[kptdobe]

First step for https://github.com/adobe/theblog/issues/663

With those changes I manage to "regularly" get a 100 for homepage and 97 with PageSpeed on my hacked CloudFlare cached instance which is not really stable. I think this is a good start, we will be able to fine tune with:

• reliable caching should stabilise the test results
• server pushes should add some speed

Author and category pages can be tackled later if really needed.

You can go there to see if everything still works as expected: https://web-vitals--theblog--adobe.hlx.page/ (which obviously will be slower because not cached)...

[kptdobe]

Homepage

Post page

[kptdobe]

Closing in favour of https://github.com/adobe/theblog/pull/669