Check for env vars at startup

[gabestein]

Motivation

So the app fails fast if it hasn't been configured properly, rather than appearing to run and then failing as it's used.

Requirements

• When the core app starts, it should check that every required environment variable exists before anything else happens.
• Converts existing process.env callsites to checking a module
  - Disallow using process.env

Acceptance Criteria

• Executes before everything else, to avoid race conditions/false positives/etc.
• Checks both front-end and back-end environment variables separately, to avoid next errors
• Adds a new module that parses process.env and exports an object with the app's parsed configuration