Infrastructure improvements

[aaitor]

After having the bot & backend running in AWS, there are a few improvements to apply:

• [x] The backend should run behind a load balancer and use https
• [x] The backend should use environment variables for configuring the github credentials
• [x] The bot should use environment variables for configuring the github verifiers
• [x] The backend should have a proper health check
• [x] The github bot should have a proper health check