Use a reserved database instance

### Context

https://github.com/AlexsLemonade/refinebio/issues/2042#issuecomment-578160214

https://aws.amazon.com/rds/reserved-instances/

### Problem or idea

Reserved instances are cheaper and we're going to need the database for a long time. Therefore we may as well save money by reserving one.

### Solution or next step

Change our infrastructure configuration to use a reserved RDS instance.

This might be a bit tricky because when we spin up a dev stack we don't want to reserve an instance for a year... We'll have to do some magic with the full_stack parameter or something...

This issue is blocked by https://github.com/AlexsLemonade/refinebio/issues/2172 because we should figure out the optimal size for our database before locking ourselves in for a year or more

Start with blocked issue 2172 to determine the optimal database size, then inspect the infrastructure configuration and the full_stack parameter mentioned here. Review the AWS RDS reserved-instance constraints and how dev stacks are created. Done means the long-lived database uses the appropriate reservation while dev stacks do not reserve a year-long instance.

Written by the indexing model from the issue text.