AMP Context and service propagation

[dvoytenko]

This is similar to property and contexts (see #48). We could place services as a map in the AmpContext or we could break them down a service-per-context. We could also consider passing services via component props.

In other words, it's either:

<AmpLightbox
    backButton={backButtonService}
    ></AmpLightbox>

vs

<BackButtonContext.Provider value={backButtonService}>
  <AmpLightbox></AmpLightbox>
</BackButtonContext.Provider>

Whether or not we use a single or per-service context: this could be a significant performance optimization since only the dependent components need to be re-rendered when a service changes.