Frontend: Remove hardcoded actions depending on content types

[bhaettasch]

e.g., in AddContentView a special Latex validator is called (which is not even using the type string from the content type class:

if content_type == 'Latex': Validator.validate_latex(get_user(request), content, content_type_data)

Replace this and other calls with generic/static methdos in the content type classes.