Adds integration tests in Github Actions

[willroberts]

Gets integration tests working in Github Actions!

Changes:

• Creates an Action which runs a Node.js container on an Ubuntu base, which enables us to add Postgres and Redis as sidecar services.
• Gets database migrations working in CI by adding missing imports and removing unnecessary Firebase access.
• Updates the imagemin-pngquant package, which was causing some build failures.
• Cleans up test scripts with namespacing, e.g. test:unit:<directory> and test:integration:<directory> so we can better target tests.
• Enables automatic integration tests for yarn test:integration:misc, which is currently the only set of tests which pass. We'll need to create a Firebase Realtime Database specifically for CI and resolve some lingering Knex .bind() issues to get the remaining tests passing.

I'll create an issue to reflect the need to fix and enable additional integration tests, and also close out #6 after adding another comment.