Set up environment variables

[simonwhatley]

This PR sets up environment variables in the service. This allows us, in future, to change the prototype set-up for UR and other reasons.

Example .env file:

SERVICE_NAME="Check if you need a lung scan"
PORT=3000

PHASE_BANNER_TAG_TEXT="Prototype"
PHASE_BANNER_TAG_CLASSES="nhsuk-tag--blue"