Upgrade to Polymer 3

[jadjoubran]

Since polymer 3 is now available
That would let us get rid of bower 🎉

• Upgrade to Polymer 3
• fix webdash serve to emulate --module-resolution=node from polymer serve --module-resolution=node
• add import "/node_modules/web-animations-js/web-animations-next-lite.min.js"?
• Extensive testing
• Plugins?
• remove all bower related stuff
• removed package.json files hack from plugins since bower is not required anymore
• Update create plugin guide

[jadjoubran]

https://www.polymer-project.org/3.0/docs/upgrade