Multiple active deployments & zero downtime assurance

- Add `saus deploy new` to spin up a new deployment instead of updating
- Add prompt to `saus deploy` that asks which active deployment to update, and if updates should be done in-place or not (ie: spin up a temporary deployment before updating DNS)
- Both of the features above will need support for multiple secret depositories
- Generate a name for each active deployment
- Probably something like `[branch]-[commit]`
- Add `saus deploy ls` for listing active deployments
- Add `saus deploy rm [name]` for destroying a deployment (with confirm prompt)
- Add `saus deploy rm --all` to destroy all deployments