[Master feature] Improve basic amp-carousel UX

[ericlindley-g]

Fix some low-hanging fruit for amp-carousel UX, including better visual and interaction design for the slide-advance affordances.

• Better advance affordance visual design (improved arrow icon)
• better "hint" affordance (currently appears and disappears without optimal timing—there could be a better pattern)

[jpettitt]

I'd love to see caption handling built in. The issue is that one slide may have a caption that's three words eg "Photo: John Pettitt" and the next will have a 25 word description. With the single height for all slides requirement on amp-carousel this becomes problematic to implement well. Typically a caption is partially shown if it overflows and amp-bind or selected used to expand it over the slide content on user action. A built in caption handler that expands on user action would greatly simplify implementation.

[ericlindley-g]

@jpettitt Good call — likely won't be completed in this activity, but adding that to the list of needs for amp-carousel