Automate deployments

[eidens]

• Automate the frontend build process. Instead of locally building the code & committing the product to a deployment branch:
  1. Build frontend code in CI
  2. Store build artifact in CI server
  3. During deployment, fetch & use build artifact from CI server
• Automate the process of deployment to the dev and prod servers.
• Trigger automated deployment to dev server after every push to master.
• Document proposed development & release workflow and clear it with other stakeholders.
• Trigger automated deployment to production server according to documented & reviewed release workflow.

[eidens]

With #248, the frontend for sdash-dev.sourcedata.io is now built and released under the tag dev during each CI run for pushes to master.

Using the altered deploy.php.example file (just add the right URL for the EC2 instance), which fetches the released frontend build from Github, the dev_server branch is now no longer needed.