Ensure serviceworker is created as /serviceworker.js (#11577)

#11538 moved the serviceworker to webbox but unfortunately
created the serviceworker in public/js rather than public/

This PR fixes this, fixing multiple issues with broken js
as a result of that change.

Signed-off-by: Andrew Thornton art27@cantab.net

.gitignore

@@ -76,6 +76,7 @@ coverage.all
 /node_modules
 /yarn.lock
 /public/js
+/public/serviceworker.js
 /public/css
 /public/fonts
 /public/fomantic