Defer actions in workflow by queueing them

[dbrumann]

Queues the generation of invite codes and sending notification emails.

In order to process these message you need to run the command messenger:consume-messages, e.g. like this:

bin/console messenger:consume-messages amqp_invite_creation --limit=1 -vvv
bin/console messenger:consume-messages amqp_events --limit=1 -vvv

This will consume one message and then exit. The messages being processed is logged to the CLI, making it easier to backtrace what is happening.