fix(systemd): Defer service enable to start_service() method #107

Job	Run time
Build charm	11m 25s
Test shared storage (mode=auto)	18m 51s
Test mode=auto	18m 58s
Test shared storage (mode=web+worker)	0s
Test mode=web+worker	0s
Publish Charm to Edge	0s
Cleanup artifacts	-1s
	49m 13s

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