Mentions in encrypted rooms swallowed by backfill are not reflected in the client

[turt2live]

As per matrix-org/matrix-js-sdk#851 (comment)

If the user goes offline and receives a substantial amount of messages, when they come back online they will almost certainly miss a bunch of events. The wrong mention count will be shown because Riot relies on counting it client-side using the visible timeline as a reference.

matrix-org/matrix-spec-proposals#1796 or similar would probably fix this.

[MadLittleMods]

Closing in favor of #17223