[DO NOT MERGE] Decentralised MLS proof-of-concept

[uhoreg]

This is juts a proof-of-concept and should not be merged. This is highly experimental, and may cause crashes, loss of data, fire and brimstone falling from the skies, cats and dogs living together, mass hysteria...

Adds Decentralised MLS encryption method.

Only for experimental use.

• Do not use on public servers
• No MLS data is persisted -- if you reload the page, you will lose your data
• Not all features are implemented. e.g. no key sharing, cross-signing, etc.
• Compatibility with future versions is not guaranteed
• Megolm may be broken
• This comes with no warranty -- may break unexpectedly

Links:

• progress tracking (incl. demo videos): https://arewemlsyet.com/
• associated React SDK PR: https://github.com/matrix-org/matrix-react-sdk/pull/10669
  • can be built with https://github.com/vector-im/element-web/commit/296d1e2b755102902283365208e7bd57d2b9ca3c -- other commits may or may not work
• hosted version: https://element-mls-proof-of-concept.netlify.app/
• Rust DMLS library: https://gitlab.matrix.org/uhoreg/matrix-dmls
• Fork of OpenMLS: https://gitlab.matrix.org/uhoreg/openmls

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Here's what your changelog entry will look like:

✨ Features

• [DO NOT MERGE] Decentralised MLS proof-of-concept (#3306).

[uhoreg]

The latest commit changes some of the identifiers. Existing rooms will break.