Can't open fresh folders with `.link` files

By "fresh" I mean folders without `.ark` subfolder. This means there is any link lacks metadata and preview. Half-initialized folders must be openable, too. In other words, user should be able to copy-paste file externally into the folder and we should catch up with all missing data.

What we do in case a link lacks metadata:
* If the link is not mentioned in `user/properties`, we query OGP and record all required fields into `cache/metadata`
* If the link also lacks data in `user/properties`, then we query OGP for missing fields and put them into `cache/metadata`
* If the link has all necessary fields in `user/properties`, then we don't query OGP

What we do in case a link lacks preview:
* We just download it and put into `cache/previews`

Start by tracing the folder-opening flow for folders without an .ark subfolder, including the handling of user/properties, cache/metadata, and cache/previews. Verify fresh and half-initialized folders open, missing metadata is filled from OGP only when needed, and missing previews are downloaded into cache/previews.

Written by the indexing model from the issue text.