Simplify import process for Cloud -> Local data imports

[rmccue]

Importing data from cloud environments into your local environment is a bit complex currently. We want to simplify this down to improve the user experience.

@joehoyle has existing work started at humanmade/altis-local-server#345 but this was for Local Server rather than generically across our local environments.

As a developer, I want to import cloud exports with a single, simple command so that I don't need to read long documentation every time.

• Running a single command (with a parameter of an export file) imports the exported data
• Database is imported and automatically search-replace'd
• Uploads are imported (i.e. via import-uploads for Local Server)
• Command works across both Local Server and Local Chassis
• Command supports backups containing data-only, uploads-only, or both
• Command outputs useful help text if used without export file parameter
• Command outputs only machine-readable output when passing --porcelain