Add import feature to sync a cloud environment to local server

Work in progress here: #345

### Description
It's a common workflow to import a backup from the cloud environment to work on locally, in particular when reproducing bugs. We can make this pretty straightforward by handling the download, unpacking and subsequent series of CLI commands required:

- db import
- search replace
- media upload
- s3 import
- cache flush
- ES reindexing

- Developers can copy an export URL Altis Dashboard and append it to the command `composer server import`. The command should be able to receive a URL or a path.

### Acceptance criteria:
- [ ] Developers should be able to import a backup with a command like `composer server import /`, with the URL being an export URL from the Altis Dashboard.
- [ ] Document the feature in the local-server docs.