Make proposal submission limit configurable per conference (#4524) #3363

Job	Run time
Check backend needs building	10s
Check pretix needs building	12s
Check frontend needs building	13s
Build backend / Build	2m 53s
Build pretix	0s
Deploy backend	42s
Wait backend deployment	2m 16s
Build frontend	0s
Deploy frontend	48s
Wait frontend deployment	11s
	7m 25s

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