add EX score to the improvements tab

[rehtlaw]

possibly either by calculating the scores live when needed, or by saving EX scores to PlayerStats.wfs.

I was gonna say we might have to make room for it, but we probably get enough space by just making the FA+ portion smaller.

[rehtlaw]

after checking out what gets saved for another feature: yeah we'd absolutely need to save EX score into the PlayerStats.