Making pagination buttons available on panels and fullbleed desktop experience.

[gmajoulet]

• Small improvement to only build the navigation buttons when needed (never on mobile)
• No page sentinel on the fullbleed desktop UI, to make sure the whole page stays clickable (it'd prevent from accessing elements beneath it, like future interactive components)
• Higher opacity on fullbleed desktop UI
• Not rewriting any CSS, just moving the pagination buttons code to their own file.
  story-desktop.css still makes a few overrides, like setting the size and position of the page-sentinel, since it's very specific to that panels UI.
• Publishers who already have their own navigation arrows will be able to hide them using the [desktop-fullbleed] attribute set on the amp-story tag.

Demo:
https://stamp-desktop-fullbleed.firebaseapp.com/examples/s20/body-painting/index.html
https://stamp-desktop-fullbleed.firebaseapp.com/examples/s20/body-painting/index-panels.html

#19270

[gmajoulet]

Friendly ping: there's actually not much to review besides the demos, since it's just moving code :) lmk if you want to talk about it in person to make it easier

[newmuis]

Not difficult to review, just forgot thanks to viewing it once in my GH notifications and never seeing it again