UISI mitigation (uhoreg's issues)

[uhoreg]

• tell other devices when you purposely don't send them keys
  • MSC. PR: MSC2399: Reporting that decryption keys are withheld matrix-org/matrix-spec-proposals#2399
  • js-sdk impl. issue: Improved communication to recipient that the sender didn't want you to see their message element-web#3831 PR: notify devices when we don't send them keys matrix-org/matrix-js-sdk#1135
  • demo
• report on olm errors. PR: record, report, and notify about olm errors matrix-org/matrix-js-sdk#1146
• report on messages sent before you joined a room. issue: Improve the UX for messages that cannot be decrypted element-web#5642 PR: Hide pre-join UTDs matrix-org/matrix-react-sdk#3881
• revamp display of UISIs
  • planning doc
  • js-sdk
  • react-sdk
• key share requests side of MSC2399
  • send info
  • receive/store info
• report on messages sent before your device logged in. issue: Hide undecryptable messages which predate your device's participation in a e2e room element-web#2258
• keyshare request feedback from server
  • MSC
  • js-sdk impl
• ...