Use core-icons icons

Remove svg assets found in core-icons
Remove a few more icons that have svg assets in core-icons
Ensure element at least imports polymer
More compatible preview url
Remove github token and support sign-in.
Remove cruft
Use the designer iframe document to create the serializer element and import elements

In ShadowDOM Polyfill, the serializer element was being wrapped, but the import nodes from meta.getImports() are
unwrapped nodes from the iframe document.

Attempting to insert them into the serializer element was throwing exceptions.
Since the ShadowDOM Polyfill cannot wrap elements created from another context, all the elements for serialization shall
be created in the designer's iframe context.

Fixes #20
fix inspector and icon
fix inspector crumbs
change palette mode to menu and fix state persistence
ensure styling is correct under ShadowDOM polyfill.
Update crumb list to have proper design host element name
Polyfill compatible :host styling.
Bind generally expects lowercase names for attributes.