[Infra] Copy old JS and CSS files to alternate storage in production deploy

[jenniferworrall]

When production code is deployed, we sometimes are missing modules because of windows still open. In this PR, add a step to copy the JS and CSS files from blue -> green or green -> blue as needed before syncing the new files to ensure that previous ones are present.

Then, if possible, delete any files older than 2 weeks from the new environment at the end of the workflow.

[alicetzhao]

Remaining: 2