Remove more namespace imports from catmaid.control package

This commit removes the namespace imports of the following modules from
catmaid.control.__init__.py: ontology, data_views, cropping. It also
makes the tagging prefixes more explicit. This is part of an effort to
make the URL configuration and the control package more readable and
maintainable. The URL configuration has been adjusted as well: The
corresponding prefixes have been extended.

I double checked, that all catmaid.control imports removed, where not in
use in other places.

This commit relates to issue #570.