Upgrade to Polymer 3

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

Start by reviewing package.json and the checklist in this issue, especially webdash serve, plugin packaging, Bower removal, and the create plugin guide. Done means completing the Polymer 3 migration, validating module resolution and web animations, testing extensively, and resolving the listed plugin questions.

Written by the indexing model from the issue text.