Consistently lowercase email addresses on the wire

[link2xt]

Trying to reproduce https://support.delta.chat/t/removed-members-and-re-join-via-qr-code/2768 I have created accounts with capitalized first letter in the email address and managed to get a message that has:

• Chat-Group-Member-Added: rae4aiv8o@nine.testrun.org header
• Autocrypt-Gossip: addr=Rae4aiv8o@nine.testrun.org; ... header
• I added member rae4aiv8o@nine.testrun.org. body.

By writing from two setups, one with capitalized letter and another without it is possible to get into a state where peerstate and contact address have different case.

Then there is a check that compares primary self address (contents of configured_addr variable) to the contents of Chat-Group-Member-Added:

deltachat-core-rust/src/receive_imf.rs

Line 1759 in acf1faf

context.get_primary_self_addr().await? == *added_addr

I have not managed to reproduce the bug with the contact ignoring being added, but it seems to be possible that Chat-Group-Member-Added contains a capitalized address while the contact thinks the address is fully lowercase and does not recognize the message as adding them. Will try to reproduce it more systematically.

Independently of reproducing and fixing the bug with the member not being added, I think we should enforce the rule that every address sent over the wire (From field, To field, envelope addresses, Chat-Group-Member-Added, Chat-Group-Member-Removed, sync messages etc.) are lowercased even if in the database we store them with a different case. This will prevent the problem from propagating between devices.

There was a related issue before: #4459

[link2xt]

PR for Autocrypt and Autocrypt-Gossip: #5032