Consider changing attachments endpoint variant behavior

[EvanHahn]

See this comment:

As a separate issue, these changes makes me think we are maybe making things difficult for ourselves by making variant optional in the attachments endpoint. I also wonder whether the observations endpoint should return all variant URLs, e.g. originalUrl, previewUrl, thumbnailUrl. When writing related frontend code it seems like this would be helpful for the non-cloud server work too.

Relates to digidem/comapeo-core#409.

We may want to return all variant URLs from the observations endpoint.