feat: Allow leaving a 'Contact Request' channel/group

[Hocuri]

A long time ago, we decided that it should not be possible to send a 'Leave' message into a Contact Request group. The reasoning was: Maybe this comes from a spammer, and sending a 'Leave' message would let the spammer know that this account exists and is read by a human. Since channels are using the same code path for leaving, it's similarly not possible to leave a channel contact request.

However:

• we're mostly using chatmail, where the spam argument carries less weight, because the spam protection works via only allowing encrypted messages (and if a spammer has your addr+key, you will likely need to change profiles, anyways - they will be able to send a lot of spam without worrying about hitting any limits).
• It won't be possible to leave unencrypted groups, anyways.
• on Desktop, there is a 'Leave' button for group contact requests, and it just doesn't do anything there

So, this PR proposes to change this logic, and to allow sending a 'Leave' message into contact request chats.

(FTR, discussion started at deltachat/deltachat-ios#2769 (comment))

Assuming we merge this PR, at some point we should adapt Android and Desktop to show this button for contact requests, but I'll write a list of things that should be adapted before making channels non-experimental, no need to do this right now.

[iequidoo]

It's ok if "Leave" is renamed to "Send leave message" or smth like this. Otherwise it can leak other metadata, not only the public key.

Also it'd good to add the reasoning from the PR description to the commit message.

[iequidoo]

Maybe split this into separate branches to not mix different cases? ProtectionBroken isn't related to leaving groups/broadcasts here

[link2xt]

It won't be possible to leave unencrypted groups, anyways.

Why? Is there any protection against this in the core?

[Hocuri]

Why? Is there any protection against this in the core?

No, only in the UIs IIRC. But for every user of chatmail-core that's not a UI (esp. bots), contact requests don't matter that much, anyway.

[Hocuri]

It's ok if "Leave" is renamed to "Send leave message" or smth like this. Otherwise it can leak other metadata, not only the public key.

I don't think we should rename this button; if we say that leaking this metadata is too big of a problem, then we shouldn't merge this PR.

Not sure which metadata is leaked exactly; it's true that it wouldn't be nice if the avatar, bio, and display name were leaked.

(BTW, this is not urgent. It has been impossible to leave contact request groups for ages, and channels are experimental, so, no need to rush this into 2.0; we can concentrate on other things first)