Need capability to set default instance-wide link preview image

Summary: There appears to be no functionality for Gitlab CE admins to change the instance-wide default link preview image for their instance(s) or the projects within them. The instance, and any projects without an avatar, receive a default Gitlab logo as their link preview image. (This image is named something like twitter_card*.jpg.) This is true even if the link redirects to the user sign-in page and the instance has defined a sign-in page logo.

Link previews to projects which have avatars display the avatar instead of the default Gitlab logo, which seems correct. However, the link preview image for pages on an instance, including hosted projects without avatars, should default to the instance sign-in/sign-up logo. Alternatively or additionally, the option should exist for instance admins to define the default link preview image for their instance.

Steps to reproduce: Use a link preview generator such as https://link-previews.stephanbogner.de/ or https://get-link-preview.vercel.app/ to examine the link preview image of a Gitlab project without an avatar in a hosted instance, or a Gitlab instance, and observe the preview image. (Suggestions: https://code.librehq.com/tris/test-project/, https://salsa.debian.org/, https://git.fosscommunity.in/, https://gitlab.freedesktop.org/)

Proposed solution:

• Link previews to projects with avatars should continue to use the project avatar as the link preview image.
• Link previews to projects without avatars should use the instance sign-in/sign-up logo as the link preview image.
• Link previews to all other pages on an instance should use the instance sign-in/sign-up logo as the link preview image.