Add grace period before firing user

### Summary
This PR creates a three season grace period to begin a new league in which the user cannot be fired. Three seasons seems like the best fit for a balance of realism and game play. Coaches typically get two seasons before their owners will consider firing them, and GMs seem to have an even larger window.

I've also added four owner messages indicating that the user would have been fired if not for the grace period. I tried to match these up to the fire messages in both style and indication of necessary improvement.

#### So the flow is...
2013: Random message
2014: Normal message or grace period message
2015: Normal message or grace period message
2016: Normal message or fired