Add method to the return the uppermost root category for a given child
category.

This will be useful for finding the root in the `current_klass` helper
in `Admin::CategoriesController`. Needed in order to not required the
`model_type` parameter for all controller actions.

We don't have a show view so lets redirect to the edit form.

This is needed as we will be adding notes to categories and after a
concrete note is saved it redirects to the notable show action.

Update the `#all_notes` method to check if the current class is Taggable
before combining tagged notes.

This will be used in the admin UI when creating Notes for a given
Category.

We're not yet showing the public link as this depends on the type of
category being used.

Will be used to allow rich text non-note styled content on the public
category pages.