[TF2] Add the unused upgrade station music

[condor00fr]

The upgrade stations you commonly go to in MvM was orginaly meant to play an "elevator tune" when stading close to it. Apparently this was cut due to being too distracting, but I feel like it deserves a place in the game, besides it is commonly used in sfm and gmod videos already so..... why not, prehaps adding a convar to enable/disable it? Still I really wanna hear it on Valve servers.

Here's what it could have sounded like, really makes you think why did such a nice touch got cut out,: https://m.youtube.com/watch?v=Q7eJg7hRvqE

[condor00fr]

Also it would be cool to make it play in main menu just like the other tracks:)