docs: make aws documentation work on reboots

Created by: ggilmore

Fixes #72

This PR improves our AWS docs by doing the following:

• Updates our recommended AMI to Amazon Linux 2
• Modifies our cloud-init script to use systemctl to make sure that the docker service (and our Sourcegraph container) starts up when the EC2 instance is rebooted
• Fixes the update instructions to make sure that the upgraded version of Sourcegraph also starts up when the instance is rebooted