Enable managed updates for Elastic Beanstalk environments

We recently switched some of our Elastic Beanstalk applications (eg. h) to use rolling / rolling with extra batch updates in order to benefit from much faster deployments at peak times (~40 => <10 minutes for h). A downside of this is that the base platform/OS no longer gets updated automatically as it does with an immutable deployment.

Elastic Beanstalk provides an automated solution for this, [Managed Platform Updates](https://aws.amazon.com/blogs/aws/new-managed-platform-updates-for-aws-elastic-beanstalk/) which perform an immutable deploy during a weekly maintenance period.

Related Slack discussion: https://hypothes-is.slack.com/archives/C4K6M7P5E/p1548940320529400

This might eventually be obsoleted by https://github.com/hypothesis/product-backlog/issues/929, but the outcome of that exploration is not certain yet.

Start by reviewing AWS Elastic Beanstalk Managed Platform Updates and the related product-backlog issue 929 to determine whether this work is still needed. Confirm which environments use rolling deployments and define completion as enabling an appropriate weekly managed platform update configuration for them.

Written by the indexing model from the issue text.