Add parse_widget function documentation
Implement Content.render_widget model method
Move widget parsing logic from view to model
Add clarifying comment
Refactor include_container -> raw widget
Move children object lookups from view to model
Remove unused field

At least I think it's unused? In either case, it doesn't *NEED* to be
"defined" here in order to be set on the object.
Move active menu-logic from context processor to template filter
Move widgets to submodule
Remove unused imports
Move widget implementations into their own classes
Merge branch 'widget-refactor' into kampanjemodul
Set admin_context to True when rendering raw widget
Add warning in editor when campaign widget has no visible campaigns