Move tracing setup URLs into own statement

This is part of an effort to make the URL configuration and the control
package more readable and maintainable. I double checked, that all
catmaid.control imports removed, where not in use in other places.

This commit relates to issue #570.
Remove superfluous importer namespace import in catmaid.control

I double checked that no importer methods are used directly through the
catmaid.control module.

This relates to issue #570.