feat(2571): DMs for desktop

[islathehut]

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[holmesworcester]

I'm trying to figure out if the "ID deterministic from participants" is a good idea. Some thoughts:

1. We can always enforce "same DM" in the frontend without making the ID the same.

2. Enforcing it in the backend by making ID deterministic from members would be helpful for concurrency (Alice and Bob both create a Alice, Bob, Carol DM) but we'll have different keys anyway, and different roles, is this too weird?

I think what happens is, right now you'll essentially have n (concurrent DM creation) different cryptographic/role contexts for the same channel id, and all messages will be visible. Is that right? That gives everyone the properties they want at this stage, or seems to!

@islathehut can you vouch for this making sense and behaving well in the concurrent case?

How do you feel about random id's and frontend-only enforcement of "same members, same DM channel"?