Include legacy Plone UI elements

- Add in the rendering of portal_css links and styles, but disable all
  default Plone styles.
Fallback styles

- Ensure the theming page is not completely mangled.
Table fixes.

- Calendar table no longer jogs.
- A table overflowing the layout will get the appropriate scrollbar.
Some more minor Plone UI corrections

- Include this sheet in the root for the unthemed layout.
- Formtab fixes.
Purge the custom code blocks.

- Sane set of defaults compatible to our needs.
Greatly simplify span rules.
Reimplement the layout switch rendering.