Notify on slack when we receive a new invitation letter request (#4315) #3152

Job	Run time
Check backend needs building	13s
Check frontend needs building	7s
Check pretix needs building	7s
Build pretix	0s
Build backend / Build	2m 39s
Deploy backend	34s
Wait backend deployment	1m 32s
Build frontend	0s
Deploy frontend	28s
Wait frontend deployment	11s
	5m 51s

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