feat(admin): add user role dialog

Admins change a user's role and see the change log. The last admin
gets a clear error when deleting their own account.

src/components/DeleteAccountCard.test.jsx

@@ -78,6 +78,18 @@ describe('DeleteAccountCard', () => {
         expect(localStorage.getItem('tbp_session')).toBe('1');
     });
 
+    it('tells the last admin to make another admin first and stays on the page', async () => {
+        deleteAnswers(json(409, {code: 'LAST_ADMIN'}));
+        renderCard();
+        openAndFill(EMAIL, 'secret');
+
+        fireEvent.click(confirmButton());
+
+        await waitFor(() => expect(toast.error).toHaveBeenCalledWith('SETTINGS.DELETE_ACCOUNT.LAST_ADMIN'));
+        expect(confirmButton()).toBeTruthy();
+        expect(localStorage.getItem('tbp_session')).toBe('1');
+    });
+
     it('shows the lock message after too many wrong passwords', async () => {
         deleteAnswers({...json(429, {}), headers: new Headers({'Retry-After': '900'})});
         renderCard();