Forbid update of title and abstract when CFP is closed (#4525) #3367

Job	Run time
Check backend needs building	12s
Check frontend needs building	9s
Check pretix needs building	14s
Build backend / Build	5m 39s
Build pretix	0s
Deploy backend	38s
Wait backend deployment	2m 35s
Build frontend	0s
Deploy frontend	36s
Wait frontend deployment	12s
	10m 15s

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