Given a `viewer: Viewer` and `mediaMessageContents: $ReadOnlyArray<MediaMessageServerDBContent`, `fetchMediaFromMediaMessageContent(...)` will query the serverDB `uploads` table and return array of "reconstructed" `Media`.
At a high level...
1. Go through each `MediaMessageServerDBContent` and aggregate `uploadIDs`
2. Query the `uploads` table for the `id`, `secret`, `type`, and `extra` of each upload
3. Go through each `mediaMessageContent` and construct corresponding `Media` by (A) pulling out information from `primaryUpload` and (B) pulling out information from the `thumbnailUpload` in the case of videos
The functionality of this function was inspired by the existing `fetchMedia(...)` and `mediaFromRow(...)` functionality in `upload-fetchers.js`... and will replace their usage in the non-legacy `multimediaMessageCreationResponder(...)`