WebR full support for dehydrated device v2

[pmaier1]

Tasks

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1. wasm bindings
2. create and rehydrate device in js-sdk
3. create and rehydrate device in react-sk
4. add dehydrated flag in Rust
5. don't show dehydrated devices as normal devices
6. periodically recreate dehydrated device
7. update MSC

[BillCarsonFr]

Adding the dehydrated flag on the device keys payload will break SQL lite store matrix-org/matrix-rust-sdk#3132

=> We have to release EAR and EIR with a new sdk version before starting creating web dehydrated device

[uhoreg]

Current status:

• dehydration only enabled by setting well-known (or if dehydration key is in SSSS)
• dehydrated device is displayed differently in device lists (if the user only has one device marked as dehydrated) -- as a localizable string in device list, and hidden in session manager but shown as an enabled feature in encryption settings
• only enabled when unlocking SSSS by passphrase/key (not enabled when verifying with another device)
• creates new dehydrated device once a week, but only if Element is not restarted/reloaded (dehydration key only stored in memory)

Work for future phases

• persist dehydration key, so that dehydrated devices can be recreated even after restarts
• add UI to enable dehydration (don't require enabling in well-known)
• gossip dehydration key?
• look into possibility of not encrypting to dehydrated device if recipient has enough other devices available

[uhoreg]

Relevant PRs:

• Add dehydrated flag to device_keys of dehydrated devices matrix-org/matrix-rust-sdk#3164 ETA Mar 18
• Add bindings for dehydrated devices matrix-org/matrix-rust-sdk-crypto-wasm#104 ETA Mar 19
• Add support for device dehydration v2 (Element R) matrix-org/matrix-js-sdk#4062 ETA Mar 21
• Add support for device dehydration v2 matrix-org/matrix-react-sdk#12316 ETA Mar 25

[uhoreg]

Screenshots:

Dehydrated device is removed from list of devices in Sessions tab.

Presence of dehydrated device is indicated in Security & Privacy tab (called "Offline device" for now)

In user info sidebar, dehydrated device is shown as "Offline device" (maybe we want to add some help text somehow?)

[pmaier1]

Thanks! I think we can find a better design for this but it's good enough for now. We should think about improvements once we plan delabsing it.

[uhoreg]

I've documented what's left to be done for phase 2 at element-hq/element-web#27336