Request preview/thumbnail only when available

[db-ot]

Initial Checklist

• I understand this is a user story and questions should be posted in the Community Discussions
• I searched issues and couldn’t find anything (or linked relevant results below)

User Story

• As a user I want that previews for all supported preview types are requested, so that I have as many previews/thumbnails as possible.
• As a user I want that for unsupported image formats no previews are being fetched so that I don't have unnecessary network requests which would fail anyway.

User Value

• Previews/thumbnails for all available formats.
• No unnecessary requests.

Acceptance Criteria

• The frontend consumes the oc:has-preview prop from the PROPFIND metadata.
• If oc:has-preview = 1, the UI displays a preview where applicable.
• If oc:has-preview = 0, the UI does not attempt to show a preview and may show a fallback.
• The hardcoded list of file types for which previews should be fetched is not used anymore
• applies to thumbnails in list/tiles view, preview in right sidebar, preview app (more?)

Depends on opencloud-eu/reva#214 being merged and brought to the opencloud repo.

[kulmann]

I've updated the user stories and user value

[AlexAndBear]

Blocked by opencloud-eu/opencloud#1133