Improve admin user flexibility

Because we have no context switching, currently admin users (defined in the ACCESS_LIST environment variable) can only see the badgekit system (though they have permissions in every system/issuer/program).  This change will allow you to add an admin user to any system/issuer/program and have them use that context instead of the default badgekit context.  They will still have admin permissions both within that context and everywhere else, but their default context will be changed.

In practice, this means that I won't have to ask people for a non-mozillafoundation.org email address to use when they want to be added to a particular issuer.