This changes to "did update" and also calls the reaction change handler to
ensure that we update the state of my reactions (to know which were sent by
you).
This displays the existing reactions a message has from all users below the
message.
Since we don't currently have an API to actually get these events yet,
adds a temporary hook that looks for a specific message to inject some sample
data. This helps build out the UI for now and can be removed once it exists.
Fixes https://github.com/vector-im/riot-web/issues/9573