configuring dashboard to run on Azure

This PR (addresses #126) includes a number of changes to allow the dashboard application to run on Azure both as an azure container instance or as a multi-container Azure Webapp.

Summary of changes:

added NGINX config file for proxying requests between front and backends, NGINX now included as a step in the frontend Dockerfile build
changed the way the frontend Dockerfile serves the React programme inline with React docs for deploying in production (although using NGINX to serve)
adjusted .js files in tasks that made calls explicitly to locahost:8000 so that calls are made relative to the host IP
docker-compose.yml adjustments
- remove network specifications (Docker connects multi containers by default to one network)
- expose ports rather than forward ports publicly for db and backend (this allows other containers to reach the ports but not make them publicly available)
- changed frontend to forward port 80, this is required for multi-container azure webapps, and means the app serves on port 80 (default port for HTTP requests)
- set images to private azure container registry used for deploying the app on Azure

This PR can be tested locally (Docker required):

$ cd CounterfactualCovid19

$ git fetch origin pull/131/head:pr-131

$ git checkout pr-131

$ docker-compose up -d

This will run the docker-compose.yml and the app will be served locally at http://localhost/

alan-turing-institute / CounterfactualCovid19