Remove service worker

[jrobbins]

While service worker is awesome, it is not needed for this app. Removing it will simplify debugging of JS and CSS issues.

This will resolve issue #892.

[jrobbins]

Demo link:
https://2020-07-28-dot-cr-status-staging.appspot.com/

[lighthousebot]

Updated Lighthouse report for the changes in this PR:

Category	New score	Required threshold
Performance	76	-
Accessibility	46	-
Best Practices	71	-
SEO	90	-
Progressive Web App	73	-

Tested with Lighthouse version: 4.1.0

[jpmedley]

What happens to all the people have installed this on desktops and home screens?

[lighthousebot]

Updated Lighthouse report for the changes in this PR:

Category	New score	Required threshold
Performance	75	-
Accessibility	46	-
Best Practices	71	-
SEO	90	-
Progressive Web App	73	-

Tested with Lighthouse version: 4.1.0

[aglaforge]

Change LGTM. My operating assumption would be that the json fetch would end up in the user's local cache. Are we properly setting the Cache-Control value so that happens?

[aglaforge]

Discussed caching offline.

[jrobbins]

What happens to all the people have installed this on desktops and home screens?

I think that the icon disappears or it becomes a plain bookmark icon. The install button was only ever offered on mobile, so I don't think many users have it.

I added to my phone's home screen and then updated the staging server. The icon is still there, but it might disappear after 24 hours. I looked through various specs and docs and failed to find what the expected behavior is.