Epic: Support Gitter sunsetting

[Johennes]

Implement MSC3946 in matrix-js-sdk:

Tasks

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1. Tests for getVisibleRooms Tests for getVisibleRooms matrix-org/matrix-js-sdk#3038
2. Factor out a function to find a room predecessor Factor out a (public) function to find a room's predecessor matrix-org/matrix-js-sdk#3039
3. Tests for getRoomUpgradeHistory Tests for getRoomUpgradeHistory matrix-org/matrix-js-sdk#3088
4. Fix the bug in getRoomUpgradeHistory (tests are cool) Fix "verifyLinks" functionality of getRoomUpgradeHistory matrix-org/matrix-js-sdk#3089
5. Refactor to use the new function for finding predecessors Refactor getRoomUpgradeHistory to use Room.findPredecessorRoomId matrix-org/matrix-js-sdk#3090
6. Adapt getVisibleRooms and getRoomUpgradeHistory to allow using predecessor events when enabled MSC3946 Dynamic room predecessors 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) Provide eventId as well as roomId from Room.findPredecessor matrix-org/matrix-js-sdk#3095

Implement MSC3946 in matrix-react-

Tasks

Beta Give feedback

1. Gitter sunsetting: Use findPredecessor in AdvancedRoomSettingsTab vector-im/element-web#24322
   T-Task Team: App Z-Gitter +3
   
2. ⭐ Gitter sunsetting: Use findPredecessor in RoomCreate #24323
3. Gitter sunsetting: ensure hidden highlights code respects MSC3946 vector-im/element-web#24324
   T-Task Team: App Z-Gitter +3
   
4. ⭐ Gitter sunsetting: Use findPredecessor in RoomListStore #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

Beta Give feedback

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:

• When viewing a room unauthenticated as a guest, it doesn't have any feature toggles enabled (like threads) #24513
• Filter out flood of member & hidden event spam when we detect the scrollback is full of it element-meta#1339
• To support accessing Gitter upload content in <img src="files.gitter.im/...">, replace external images with alt text instead of completely stripping them out #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.