Contact for MemberAddedToGroup and MemberRemovedFromGroup should not be looked up by address

[link2xt]

There is a TODO item in the source code:

core/src/mimefactory.rs

Lines 1358 to 1362 in 416131b

	SystemMessage::MemberAddedToGroup => {
	// TODO: lookup the contact by ID rather than email address.
	// We are adding key-contacts, the cannot be looked up by address.
	let email_to_add = msg.param.get(Param::Arg).unwrap_or_default();
	placeholdertext =