Prevent delete of last admin user

cps/admin.py

@@ -650,10 +650,16 @@ def edit_user(user_id):
     if request.method == "POST":
         to_save = request.form.to_dict()
         if "delete" in to_save:
-            ub.session.query(ub.User).filter(ub.User.id == content.id).delete()
-            ub.session.commit()
-            flash(_(u"User '%(nick)s' deleted", nick=content.nickname), category="success")
-            return redirect(url_for('admin.admin'))
+            if ub.session.query(ub.User).filter(and_(ub.User.role.op('&')
+                                                             (constants.ROLE_ADMIN)== constants.ROLE_ADMIN,
+                                                         ub.User.id != content.id)).count():
+                ub.session.query(ub.User).filter(ub.User.id == content.id).delete()
+                ub.session.commit()
+                flash(_(u"User '%(nick)s' deleted", nick=content.nickname), category="success")
+                return redirect(url_for('admin.admin'))
+            else:
+                flash(_(u"No admin user remaining, can't delete user", nick=content.nickname), category="error")
+                return redirect(url_for('admin.admin'))
         else:
             if "password" in to_save and to_save["password"]:
                 content.password = generate_password_hash(to_save["password"])