✨Adds button when new update is available

[Enriqe]

Closes #21721
Master issue: #21714

When button is clicked, user is taken to the latest page in the story.

[gmajoulet]

The code mostly LGTM, but can we check with product/UX/interested publishers that this is the expected behavior before checking in the code?

[Enriqe]

Thanks @gmajoulet, this behavior was signed off at the design phase and again at a UX meeting last week :)

EDIT: It's also important to note that the exact UX of the button is still in progress, so we will update it when it's ready. (As noted by the TODO comment in the css file).

[newmuis]

Sorry, I'm not sure I understand from reading the code: does clicking the button take you to the last page? or the first of the added pages? I would expect the latter...

[Enriqe]

As chatted more offline: it takes you to the last page, because we want to adhere to the amp-live-list behavior, as well as the behavior of most live blogging platforms, which take you to the "latest" entry. UX was also part of this discussion and agreed with this.