Adds disabled Postgres ECS service

[willroberts]

Summary

I partially implemented Postgres on ECS, but the rexray/ebs and portworx Docker volume plugins haven't been updated since 2019, which has me reconsidering RDS.

Since I ended up doing part of the work, and adjusting our Terraform modules to support more features, I've included that cleanup in this PR as well.

Infrastructure changes (.github/, terraform/, etc.):

• Disables the Postgres ECS service (still using RDS)
• Adds support for volumes, mount points, and placement constraints for ECS services
• Adds a disabled IAM policy which allows EC2 instances to manage EBS volumes
• Fixes whitespace in IAM policies

Testing

Have you have tested your changes in the following scenarios? Feel free to check off scenarios which don't apply.

• [x] Starting backend services locally with docker compose up succeeds.
• [x] I am able to create a new user and log in locally.
• [x] I am able to complete a practice game locally.
• [x] I am able to complete a purchase of Orbs, etc.