improve async blocks performances (tabs+accordion)

[ramboz]

The page https://main--delta--hlxsites.hlx.page/us/en/skymiles/how-to-earn-miles/skymiles-partners-offers has relatively bad performance compared to the rest of the Franklin pages. The cause seems to be mainly that the async blocks we use (accordion + tabs) await all the content load and decoration before returning from the decorate method.

Ideally, we should refactor the code so that:

• accordions do not await on their content, since it is not visible
• tabs only await the visible tab, and not all of them

Furthermore, we can probably explicitly delay the rest of the "hidden" elements decoration by using one of following options:

• defer the decoration to the end of the event thread via setTimeout, so we offset the load on the main rendering thread for the rest of the phase
• defer the decoration until the element is actually in the viewport, using an intersection observer
• defer the decoration until the element gets keyboard focus or has the mouse over it, so we can load and decorate hopefully in time before the user clicks the tab/accordion to avoid a flash of content