Add support for official collections

Description

It would make sense to have official collections that can be synchronised from upstream.

Possible implementation

This would involve:

• Making the user_id nullable
• Adding synchronisation support from the API (perhaps with a flag)
• Handle null-safe collection users (show an "Official" badge)

This would also need to have collections upstream, suggesting the format:

[
  {
    "id": "...",
    "name": "Collection Name",
    "description": "...",
    "keeps": [
      "uuid-1..."
    ]
  }
]

Having thought about this more, it would likely be better as a idempotent seeder as collections are Keep-specific.

Other considerations

• It could also be good to have a "Clone" functionality, where you can clone official collections