context variable "body_css_classes" to append context specific css

classes to the body tag. The default classes are:

  * logged_in|not_logged_in,
  * instance_key_<key>,
  * global_nav_home|global_nav_instances,
  * added_static_content|context_static_content,
  * css_classes from the static pages backend.
Make sure the css class active_global_nav_<id> and context variable active_global_nav stay in sync
Use 'body_css_classes' instead of 'active_subheader_nav' to genrated controller specific css classes (area-<id>)
Make filesystem and db staticpage backend compatible with css_classes
fix c.active_subheader_nav

fix compability issues introduced in "Use 'body_css_classes'
instead of 'active_subheader_nav' to genrated controller
specific css classes (area-<id>)"
Merge pull request #772 from liqd/feature-add_context_specific_css_classes+1

body_css_classes (rebased against develop)