Fix service-worker
It seems to be working as expected (Not sure what its supposed to do), but linter complains:
3:1 error 'workbox' is not defined no-undef
4:1 error 'workbox' is not defined no-undef
5:1 error 'workbox' is not defined no-undef
24:1 error 'workbox' is not defined no-undef
26:20 error 'workbox' is not defined no-undef
49:24 error 'workbox' is not defined no-undef
52:11 error 'workbox' is not defined no-undef
59:16 error 'workbox' is not defined no-undef
81:21 error 'workbox' is not defined no-undef
91:1 error 'workbox' is not defined no-undef