Add retirement mechanics

[stefan-brus]

Ideas:

• At year 47 (when the player character has spent 65 years of life, so 65 - 18), a "retire" button should become available.
• Retirement should make the player unable to take new jobs, lose their job, and perhaps lose the penalties from career upgrades.
• An option to retire in advance should become unlocked at some point, based on either income, capital, or passive income (TBD)