Backport `admin_or_auditor?` to CE

Currently, most times I add a new user.admin? site to CE, I'll have to remember to add an EE MR changing it to admin_or_auditor?. This seems error-prone and likely to break things often.

Let's have an User#admin_or_auditor? (and perhaps even User#auditor?) method in CE that we can use directly, and just change the implementation of that in EE instead.

Thoughts @DouweM ?