Rollback assets, if possible, when running a deploy rollback

[martinemde]

Ideally, users that decide that rollback is an acceptable deployment strategy for their application will choose to use the asset_strategy: private. The private strategy stores assets in the release directory and does not share or move assets, allowing rollback to function correctly. This is supported already in the current version.

However, if the default shifting asset_strategy is used, we could potentially support one level of rollback.