check-in assets, making build step optional

This pull request checks in the assets for record-crate, so that the server can be run without `bower` or `sass`.

* This makes the npm package able to be run in isolation.
* if someone wishes to contribute to the project, there's now a separate `npm run build` which can be used to re-generate assets, .e.g., `css` from `scss`.

CC: @DeviaVir