Not being able to send messages in a group if Autocrypt-Gossip was lost

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Delta Chat distributes the keys using Autocrypt and Autocrypt-Gossip headers.

Autocrypt header is sent in every message, so when you receive a message
from someone, you always have their key, and it is nearly impossible
to have no key for someone in 1:1 chat unless it's a chat with webmail or Thunderbird.

Autocrypt-Gossip headers are however only sent when new member is added
or after 2 days (gossip_period config). This means if a new member is added,
but someone has not received a "member added" message,
they don't have the key for the new member.
For added member it's even worse, such new member
possibly does not have the key for many group members.

If the group is not protected,
an user missing the key can still send unencrypted message.

But if the group is protected,
users get an error
"proper enc-key for ... missing, cannot encrypt"
and the message is not sent at all.
User cannot even leave the group.

Possible solution is to send the message
to every contact that has a verified key
and for others either don't include them in recipient list
or send the message that they will not be able to decrypt.
The error should be still displayed on the sender side.

Related issue: #4828