Display private sets for admins when fetch /all

[LarsFlieger]

As of now fetching the sets from the public endpoint with an admin account will return every set (including private).
This hinders the ability of the admin account to play the game normally as private sets are not relevant for the admin when playing normally.
A solution to this issue would be to introduce a query parameter that allows for specifying the desired visibility:

api/sets?visibility=all => private/public when user is admin
api/sets?visibility=private => private when user is admin
api/sets?visibility=public / api/sets => display public => doesn't matter is user is admin

default is public