load footer columns from staticpages if present

The default footer is (in most parts) translatable. However for many of our projects we want to customize the footer. We usually do that using diazo. While this allows us to customize we loose translation.

This PR allows to define footer columns in staticpages. This way we get translation and customization at the same time. If a footer column does not exist this falls back to the original template.

To use this remove the relevant rules from `rules.xml` and create `footer_1`, `footer_2` and `footer_3` in your staticpage backend (e.g. kotti).