fix(sw): handle dev serviceworker 404

[zetlen]

Modified src/index.js to only register a service worker if process.env.SERVICE_WORKER_FILE_NAME is truthy.

Modified webpack.config.js to set process.env.SERVICE_WORKER_FILE_NAME to false in development mode, unless the special ENABLE_SERVICE_WORKER_DEBUGGING env var is truthy.

Now, service workers are only generated OR requested:

• in production
• when they are directly under development, indicated by the env var ENABLE_SERVICE_WORKER_DEBUGGING.

Fixes #64.

[zetlen]

This one is right in the middle of @jimbo's workflow, so we'll wait for him to approve before merging.

[DrewML]

@zetlen with @jimbo out for a week, would prefer to land this and we can handle the conflicts.

[magento-cicd2]

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