Epic: Support Gitter sunsetting

[Johennes]

Implement MSC3946 in matrix-js-sdk:

Tasks

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1. Tests for getVisibleRooms matrix-org/matrix-js-sdk#3038
2. Factor out a function to find a room predecessor matrix-org/matrix-js-sdk#3039
3. Tests for getRoomUpgradeHistory matrix-org/matrix-js-sdk#3088
4. Fix the bug in getRoomUpgradeHistory (tests are cool) matrix-org/matrix-js-sdk#3089
5. Refactor to use the new function for finding predecessors matrix-org/matrix-js-sdk#3090
6. Adapt getVisibleRooms and getRoomUpgradeHistory to allow using predecessor events when enabled matrix-org/matrix-js-sdk#3042
7. Add to the API the ability to get the tombstone event ID as well as the room predecessor ID (rename it findPredecessor) matrix-org/matrix-js-sdk#3095

Implement MSC3946 in matrix-react-

Tasks

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1. Gitter sunsetting: Use findPredecessor in AdvancedRoomSettingsTab vector-im/element-web#24322
   T-Task Team: App Z-Gitter +3
   
2. ⭐ #24323
3. Gitter sunsetting: ensure hidden highlights code respects MSC3946 vector-im/element-web#24324
   T-Task Team: App Z-Gitter +3
   
4. ⭐ #24325
5. Gitter sunsetting: Use findPredecessor in RoomNotifs vector-im/element-web#24326
   T-Task Team: App Z-Gitter +3
   
6. Gitter sunsetting: Use findPredecessor in EventTileFactory vector-im/element-web#24327
   T-Task Team: App Z-Gitter +3
   
7. Gitter sunsetting: Add a labs flag for MSC3946 support and propagate it vector-im/element-web#24328
   T-Task Team: App Z-Gitter +3
   

Other

Tasks

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1. Gitter sunsetting: Pass feature flag through to everywhere we use getVisibleRooms and getRoomUpgradeHistory vector-im/element-web#24417
   A-Room-Upgrades O-Occasional S-Minor T-Defect Z-Gitter +5
   
2. Gitter sunsetting: Can't go back to previous room version if I've never been there vector-im/element-web#24453
   A-Room-Upgrades A-Timeline O-Frequent S-Major T-Defect Z-Gitter +6
   
3. Gitter sunsetting: Adapt implementation to make use of via_servers property in m.predecessor event vector-im/element-web#24611
   +0
4. Gitter sunsetting: A room with lots of hidden events is slow vector-im/element-web#24480
   A-Performance O-Uncommon S-Minor T-Defect Z-Gitter +5
   

De-scoped for now:

• #24513
• element-hq/element-meta#1339
• To support accessing Gitter upload content in <img src="files.gitter.im/...">, #4044

Internal references

myhours: https://app.myhours.com/#/projects/1994529/overview

[Johennes]

Descoping #24513 as it's fairly low impact.

[Johennes]

Also descoping element-hq/element-meta#1339 since there is no clear general solution available right now and we have the general performance improvement of #24480 in scope.