Undeliverable group message - Changed RSA keypair

[ChristianeMussmacher]

Reproducible on all iOS and Android devices

How to reproduce:

• iOS or Android Admin creates Group
• Group Members join group
• All Members are offline
• iOS / Android admin renews RSA keypair
• iOS / Android admin sends message to group

--> all messages are undeliverable (red message bubble) until one group members is online again and new keys are exchanged

[pavel-mayer]

Needed a fix on the server that should also fix other group key problems: hoccer/hoccer-talk-spike@722eb6b84c45c48f37e6226b888ca52517a2bde8