Support for specifying custom files for small resolution images for avatars (and similar)

It would be nice to have a way for users to provide hand-crafted small resolution images for icons for things like flair, room avatars, even user avatars - so that when these avatars are used as favicon-style thumbnails, we have a pixel perfect avatar rather than a bluntly scaled down version of a larger image.

It feels to me like this is something that the media repo API should support, so that a user can upload alternative images for a given URL to 'seed' the lower resolution variants.

Start by reviewing the media repo API and the existing behavior for avatar URLs used as favicon-style thumbnails. Determine how alternative, hand-crafted images would be uploaded, associated with a source URL, and selected for lower-resolution variants. Done means the API behavior and compatibility expectations are specified clearly enough to implement and test.

Written by the indexing model from the issue text.