Configure development/production/testing environments not just for the database

[amcewen]

At present api/knexfile.js contains the way to switch between different environments: development, testing and production.

For #7 we'll want to add different email server details for each environment too (I think each developer will have their own email username and password, which would be set in .env) so we'll want something more generalised rather than just in the database code.

[amcewen]

https://stackoverflow.com/questions/64225631/how-can-i-add-production-development-and-test-environment-in-node-js looks like it might be a sensible way to approach it