Themes should extend sortable-table to avoid external css files

[MetaMemoryT]

Currently polymer does not handle external css well, you must declare it in the head of a document, and this breaks encapsulation of web components.

We could extend sortable-table into sortable-table-bootstrap, sortable-table-ext4js, and have the theme embedded in <style shim-shadowdom> elements. The only frustration of that is auto-formatting embedded css, but js-beatify and atom-beautify help with that.