Restrict by class

[DamienVG99]

Restrict entry of a board by class.
This means, if someone from year three tries to enter a board, which is meant to be for year 1, they can't.

In the best possible way: Disable the button in the frontend
In the wordt way: Make them click it, but give a error message stating they cannot enter the board, because they are not in the right class.

[Feddman]

@DamienVG99 Pull request season is opened!

[DamienVG99]

@Feddman i'll take a look this evening 😉

[D252874]

this is actualy the same as issue #14 so there was an idea back than but nobody looked at it?