Implement user permissions with `cancan` gem

[f3ndot]

"Anonymous" Users:

• "Anonymous" users can create incidents, witnesses, cars.
• "Anonymous" users cannot edit incidents, witnesses, cars
• "Anonymous" users cannot update user accounts.
• "Anonymous" users cannot set a score override.

Registered Users:

• Users can create incidents, witnesses, cars.
• Users can update incidents, witnesses, cars that belong_to them.
• Users can update their own account, but not others.
• Users cannot set a score override.

Moderators:

• Mods can create incidents, witnesses, cars.
• Mods can update incidents, witnesses, cars that belong_to them.
• Mods can set an override score for ALL incidents.
• Mods can update their own account, but not others.

Administrators:

• Admins can create incidents, witnesses, cars.
• Admins can update ALL incidents, ALL witnesses, ALL cars.
• Admins can set an override score for ALL incidents.
• Admins can update ALL accounts.