move print_progress logic from users.py to skills.py

I originally had the print_progress function in users.py because it made
sense for a user to be able to print her own progress. Upon thinking
about it, I feel it makes more sense for a skill object to print out its
own progress, especially since it's already calculating its own level
and progress. So I moved the calculations and formatting/printing over
into skills.py and users.py's print_progress function just loops through
the skill objects and calls its print_progress function.