Admin permission bug

Describe the bug
is_admin does not grant permission to access the Admin interface.
If a user is only is_admin they encounter an error (see below). If a user has both is_admin and is_staff, they can access the admin interface, but it informs them they do not have appropriate permissions.

This was tested on both path mode (/doctest) and a single journal install (Playtest). Based on testing, only is_superuser can access the Admin interface. It makes is_admin currently not a not particularly practical role.

(What is_admin does appear to allow is for is_staff to be able to see that they are staff and untick that box. When a user only has is_staff they cannot see that they are staff or untick it. is_admin can give themselves and others is_staff. Not sure if that is intentional? I assume so as I think they are meant to be able to access the admin interface. Again, tested on both options, is_admin by itself cannot access the press level manager.)

Note: tested by logging in as users as well as controlling them, to be absolutely sure.
Janeway version
1.7

Screenshots
User is_admin

'Admin does not show on menu dropdown on front page'

User is_staff and is_admin

'Admin' also shows in menu dropdown on front page.