member added races in verified chat causes issues displaying message from new member

[adbenitez]

The problem

A and B are on server A1 and B1, delivery is fast between them,
C is on server C1, delivery to B1 is fast, but to A1 is slow
C creates verified group with A, they chat there, then C adds B,
B receives the invitation immediately and sends "hi",
at that point A still doesn't get the member-added from C but already gets the "hi" from B, for A, B is not a member yet so message is displayed as "unknown sender" if it is an image or webxdc app is not possible to use it

Reproduction steps

it is easy to reproduce:

• create verified group with A account,
• add B,
• A goes offline,
• B sends message,
• A removes B and goes online,
• A receives B's old message but B is no longer member, message is "from unknown sender" and can't be used

Proposed solution

instead of making the message totally unusable, just display the sender as "overriden sender" (~ prefix on the sender name) and mark the message with an error (gets red exclamation mark (!), it is more common it is a sync issue than an "attack" if it is really an attack anyway the attacker can't receive any replies, just send messages, and they will marked with errors, on tap user can see the explanation we currently put in the message bubble or so

[link2xt]

There is a #4999 for roughly the same problem.

[adbenitez]

lets move there

[link2xt]

This is fixed by #5024