Enable multiple admins

[TiBiBa]

Idea incl level Currently we only enable one admin within our environment as this is set as a string and we verify if the current user is the same as the username set in the environment. It would be nice if we changed this to a list to enable multiple users to be an admin. However, it is up to @Felienne if this is preferable. We can also change it to a list and still keep only one user in the list, but this at least makes it more scalable for the future.