Perks effect on maintenance

The skill level gap between the perks of the kerbal and the requirements
now affects the maintenance bonus given during maintenance.

Fixed a logic bug that made the perks check fail when the requirements
list is empty.
Improved perks upgrade button

If the kerbal cannot be trained (either applicant or assigned), the
current perk level is displayed nontheless. Instead of that terrible
label, a disabled button is displayed instead of the upgrade button.