Element R: fix `isCrossSigningReady` not checking identity trust (#4156)
* Fix inconsistency between rust and legacy
* Add tests
* Review: better comment
Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
* review: Better doc
Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
* Simplify test data and some comments
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Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>