Institution admins can't see or delete other institution admins

[phavekes]

A institution admin who received this role via an invitation (rather than through the SAB role) must also be searchable and removable. Do not delete the entire user account (they may have other roles and permissions); only the institution‑admin role should be removed.
Currently, institution admins are neither displayed nor searchable in the user list. This will not change see #649 (comment) for and explanation.

---

Tasks

• Display institution admins of the same institution and all Application Managers with role access belonging to the same institution in a new page accessible from top-right user menu.
• Add a delete button for those institution admins who became this by invitation and application managers
• Prevent self‑removal: a user cannot delete their own institution‑admin role. Additionally, an institution admin who obtained the role via an SAB role cannot be removed through this mechanism.

[oharsta]