Refactor user, skill and cert data into separate modules

This is for https://github.com/eve-val/evecerts/issues/4.  No new functionality is being added and so this should behave exactly as it does today.

Some of the changes don't really simplify main.py very much (trade one line for another), but  some of the code gets re-used by other modules and I didn't really want a circular dependency on main.  I also couldn't think of a good way to consistently use dicts in lieu of model instances on one side of the split versus the other.  Feedback welcome.