Make ‘pages’ modular apps.

[alloy]

From #19

For bigger projects I'd encourage a modular architecture. E.g. what we do in Force/MG where you have sub-apps and a components directories that make it easy to say "fork" the artist page by copying the sub-app and splitting between the two at a routing level. The co-location of files also makes it easy to experiment and drop old code. I also think this provides obvious "escape hatches" for when one finds themselves fighting the React/Relay/etc. stack, e.g. they could implement a sub-app that does more vanilla Express stuff or introduces a new approach to UI suited to that edge case need.

[orta]

We'd also want to consider about how this can feel similar-ish in React Native