Pull functionality out of `fetchMediaFromMediaMessageContent(...)` so it can be reused in `rawMessageInfoFromRows(...)` to reconstruct `Media` objects for `MULTIMEDIA` messages.
The difference between the two is that in `fetchMediaFromMediaMessageContent(...)` is we're fetching the upload rows whereas in `rawMessageInfoFromRows(...)` they're given to us.
In next diff we'll consume this function in `rawMessageInfoFromRows` to correctly pull `MULTIMEDIA` messages from the DB given the the `content` column of the message (stringified MediaMessageServerDBContent` in addition to the upload rows.
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Depends on D5191