Feature: Functions bearers

[Bibo-Joshi]

Like CEO, CFO, Notenwart, Internetwart etc. Some thoughts:

• Conductor should then probably move from Instrument to function
• As the defined functions can change yearly, they should not be hardcoded like the instruments
• Maybe it would be best if the users don't set the functions on their own, but the admin does each year after the general assembly
• Alternatively, the admin could be required to verify the users changes