go: small refactor of http code, build pipeline

[roobre]

This will be the first PR in a series that aim to give the Quickpizza application two deployment modes:

1. A single service handling all endpoints, exactly as it is now. This will be the default mode.
2. A new mode splitting the API into several deployments, i.e. in a microservice architecture.

The goals for 2 are:

• There will be one single build pipeline for the application, which will produce a binary/image capable of deploying in both modes.
• Microservices-style deployment will be achieved by running different deployments of the same image, each with certain endpoints enabled/disabled in runtime.

Towards this goal, this PR adds runtime, env-var based switches to control whether certain endpoints are enabled, and tweaks the build pipeline a bit.

Subsequent PRs will further split the service and add env-var based config options so multiple services can find each other.