In the past, the input to `multimediaMessageCreationResponder` included the `threadID`, `localID`, and `mediaIDs`.
`mediaIDs` contained the list of upload IDs that belonged to the given multimedia message being created. This worked fine under the assumption that there was a 1:1 relationship between media and uploads. However, we break that assumption with video messages...
Going forward we will need to support a 1:many relationship between media and uploads. Specifically with video messages we have 2 uploads (actual video and thumbnail image) for a given video.
The problem is that we need to communicate this more complex relationship between media and uploads to the `keyserver` when creating a media message. The way we're going to do this is by modifying `multimediaMessageCreationResponder` to also accept a `mediaMessageContents` (of type `MediaMessageServerDBContent`) parameter that encodes the relationship between messages and uploads.
This diff just modifies the validator to support both types of inputs.
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Depends on D4990