[DO NOT MERGE] DMLS proof-of-concept

[uhoreg]

This is just a proof-of-concept and should not be merged. This is highly experimental.

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

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Add one of: T-Deprecation, T-Enhancement, T-Defect, T-Task to indicate what type of change this is plus X-Breaking-Change if it's a breaking change.