Refactor app.js into per-component routes where appropriate

`build-system/server/app.js` is [swelling](https://github.com/ampproject/amphtml/pull/25100#discussion_r341291831) in size. The good news is `build-system/server/routes` holds per-component routes. This issue is for tracking the effort of identifying any relevant endpoints in `app.js` attributed to specific component usage and pulling them into appropriately labeled files in the `routes` directory.