add field to GSR for the number of days a GSR is bookable in advance. #528

Job	Run time
Backend Check / Django Check	1m 56s
Frontend Check / React Check	36s
Frontend Check / Publish frontend	2m 6s
Build backend	1m 58s
Build frontend	2m 14s
Publish Images	0s
Deploy	0s
	8m 50s

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