* strip mentions from forwarded messages
fixeselement-hq/element-web#30883
* call attachMentions() for empty m.mentions in forwarded messages
As there is no EditorModel, attachMentions() currently does nothing
* fix lint and typecheck
* update test "should be navigable using arrow keys"
* update test "forwards pin drop event"
add empty mentions to expected content
* add doc to transformEvent() & elaborate on attachMentions()
* add test "strips mentions from forwarded messages"
* fix lint
* update source of `attachMentions()` import
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Co-authored-by: David Baker <dbkr@users.noreply.github.com>
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