Element-R: implement device verification and cross-signing

[richvdh]

There are a few aspects to this.

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1. Element-R: implement Secure Secret Storage #24982
   A-Element-R T-Enhancement +2
   
2. Element-R: wire up device lists #24833
   A-Element-R A-Session-Mgmt O-Frequent S-Tolerable T-Enhancement +5
   
3. Element-R: Replace CryptoBackend.checkDeviceTrust #25092
   A-Element-R T-Task +2
   
4. Element-R: replace uses of checkDeviceTrust with getDeviceVerificationStatus #25093
   A-Element-R T-Task +2
   
5. Element R: set up cross signing during registration #25314
   A-Element-R T-Task +2
   
6. Element-R: login-and-verify tranche 1: groundwork #24983
   A-Element-R T-Enhancement T-Epic +3
   
7. Element-R: login-and-verify tranche 2: rust-backed implemention of VerificationRequest #25316
   A-Element-R T-Task +2
   
8. Element-R: login-and-verify tranche 3: completion #25319
   A-Element-R T-Task +2
   
9. Element-R: get the cross-signing settings panel working #25318
   A-Element-R T-Task +2
   
10. Element-R: Handle incoming verification requests #25320
    A-Element-R T-Task +2
    
11. https://github.com/vector-im/crypto-internal/issues/124

[richvdh]

This seems to have been done months ago!