Implement logging

This will log when levels or character classes is added to the
character. This is done via a association callback on the has_many
association on the character object.
Implements Character#levelup_state

Adds a levelup_state field to the character, and builds a statemachine
around it, to allow the character to move through the levelup progress.
Refactor the levelup progress into a partial

This moves the levelup progress menu into a partial, to it can become
general for the entire process.