Share buttons: Add option to make sticky

[laurelfulford]

We've had a request to make the social share icons sticky, similar to the New Yorker: https://www.newyorker.com/news/daily-comment/ayman-al-zawahiri-and-the-taliban

In that case, the placement is to the left of the story; I'm not sure offhand if we can make them visible on scroll with AMP like that example, but it's testing to confirm (if I remember right, AMP seems to have more limitations around changing an elements overall appearance, like size, because it can shift other elements -- changing opacity, and therefore visibility, does work).

This is doable with a third-party plugin (like AddThis) but it would be nice to have something "baked in" so it's not necessary to install another plugin to achieve this.

Somewhat related to this issue: #1531 -- though it will probably be easier to make them a "freestanding" element rather than trying to work them into all possible header configurations.