Add `baseurl` to the default theme

The baseurl needs to be added to the following files so that the theme will work with GitHub Pages.

- [x] add `baseurl` to `_includes/head.html`
- [ ] add `baseurl` to `_includes/sidebar.html`
- [ ] add `baseurl` to `_layouts/post.html`
- [ ] add `baseurl` to `index.html`