WIP: refactor: Unify naming of direct/single/1:1 chats

[j-g00da]

draft

Closes: #8368

[r10s]

renaming documentation and internals is fine.

but renaming existing external api is a different thing - that tends to cause troubles on many ends, that are not foreseeable yet. we had that in the past as well, where we also thought it is a simple change ... good apis are stable - and there are loooots of inconsistencies wrt naming, not just that one, we should not not open that can of worms :)

[Hocuri]

Yes, the API will need to stay as-is for now, because coordinating with all clients is a lot of effort. Probably it still makes sense to internally use the name "private chat", but alternatively we could decide for "single chat" because this is what the APIs are using.

BTW, to explain the social practices to you @j-g00da: we're usually giving @r10s and @nicodh a veto right over any API changes, because they are the ones who need to oversee the adaption of the UI implementation (@r10s for Android and iOS, @nicodh for Desktop, even if they're not always implementing it themselves). (Of course a veto right doesn't mean that we can't discuss.) That's why I requested a review from @r10s on #8408 and #8409. If the API change was less straightforward, I would also have requested a review from @nicodh.

[j-g00da]

Yeah I wondered if we really want to break the API, this is a lot of breaking changes.

[j-g00da]

So we have two options, either internally rename to "direct" but leave "single" on the API boundary or rename everything to "single".

I really prefer "direct" as it fits better, but I think for consistency it makes sense to just rename everything to "single"...