Category form adds category field even if there are none

[slomo]

The categories form could only add the category field if there are any categories to choose from. So the field can also be make required removing the need to set the empty_label to None. Also the show_categories helper could go and be replaced by {% if form.category %} .

For OPIN there is also the need make categories optional. Either the form should have a property. An alternative maybe an method that will be passed the module, so we can decide on a per module basis if categories are optional.

[slomo]

FYI: For now we have implemented some hack in OPIN to get the optional categories working.