Improve speed with service worker

[jadjoubran]

Sw registration temporarily disabled until figuring out plugin structure and reaching maturity

[jadjoubran]

Also need to have a toast to reload on newer version

[jadjoubran]

• [x] enable service worker registration in index.html
• [x] copy service-worker to build folder in polymer.json
• [x] switch to workbox and manually generate service worker on build step
• [x] delete old configuration if not needed anymore
• [x] test locally & make sure service worker is efficiently caching all required resources
• [x] integrate paper-toast on update
• [x] express should disable cache headers on service-worker.js
• [ ] due to the nature of the feature (only available in build folder), try it out over npm link or a beta release

[jadjoubran]

Can be tested with 1.4.0-0 pre-release, tagged next npm install --save-dev webdash@next

[jadjoubran]

service worker can be tested with npm install --save-dev webdash@next (v1.4.0-1 pre-release)

[jadjoubran]

• [ ] toast should always have higher z-index than plugins on hover