Support offline messages

Send a message with device offline or server down. Check that in both cases the message is stored but not sent. Check that you can send more than one message. Check that you can delete offline messages. Send at least 1 message being offline. Go back online and check that the messages are synchronized immediately. Now send messages with server down. Put the server up again and, in the app, access the discussion. Check that the messages have been sent. Send messages with server down again. Put the server up again and wait. In 5 minutes or less the messages should be automatically synchronized. Now send offline messages to a user that has blocked you. Synchronize them while being in the discussion and check that an error is shown.