This commit introduced a big breaking change of changing Author and
Namer to no longer have any reference to Icon or Image containers and
interfaces.
Instead, in the case of Author and Namer, it relies on the label being
updated by either an update setter or LabelContainer. The frontend
should get the first image/avatar to display that instead.
This commit also added ReadIndicator and related interfaces to support
the read receipts feature seen in Matrix, Telegram, Messenger and co.
The UnreadIndicator interface was broken to add the MarkRead method,
which hands explicit control of setting read messages for the current
user to the frontend instead.
diamondburned
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