Update backend Message type to add "visibleToUids" array to enable re-rendering messages after changes

[h1divp]

What happens if a user edits or otherwise updates a message in any way shape or form?

This could include changing message content, replying to a message, whispering, changing their display name, etc. But this would entail needing a way for, after an update is made, to re-render the message in question to all of the users who already see it.

I believe that this should be done by storing the UIDs of the users that the message is sent to within the message (generated by the getNearbyUsers() function), so that they can be re-used later with other socket API endpoints.

Please make this type change. This array attribute can be optional (using the optional property accessor ?.) as many functions probably don't need it. This would also save yourself some time from having to update a bunch of functions to match the new type.

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[aaditkamat]

Since #183 requires this, I thought I'll update the Message type first