Upgrade to Uvicorn 0.10

[tomchristie]

• We can now drop the --proxy-headers flag, since we'll use the ALLOW_FORWARDED_IPS environment variable, which Heroku sets.
• We'll now be using multiple workers, based on the WEB_CONCURRENCY environment variable.
• Our ./scripts/run loads the .env file, so there's no need to have the Heroku toolkit installed locally to run the app.
• We've got lovely colourized log output.