Fall back to receipt timestamp if we have no event (js-sdk part)

[andybalaam]

There is similar logic in matrix-react-sdk, so we change that in matrix-org/matrix-react-sdk#10974

Part of element-hq/element-web#10954

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Here's what your changelog entry will look like:

🐛 Bug Fixes

• Fall back to receipt timestamp if we have no event (js-sdk part) (#3400). Contributed by @andybalaam.

[andybalaam]

Not doing this for now because:

1. The problem I am finding right now is to do with stuck unreads, whereas this is about notifs, so Fall back to receipt timestamp if we have no event (react-sdk part) matrix-react-sdk#10974 is more relevant
2. With any luck, the homeserver will give us the correct count for notifs, so if we make sure we believe that, it may be more effective than guessing as we are here
3. If we do want to guess within matrix-js-sdk, we should return an enum with e.g. Read, Unread, ProbablyUnread, ProbablyRead or similar to be clear that we are using heuristics.