add "sudo mode": confirm password before doing sysadmin work

tests/ui/test_sudo_mode.py

@@ -0,0 +1,24 @@
+from tests.common.common import ADMIN_USERNAME, ADMIN_PASSWORD
+
+def test_sudo_mode_required(admin_browser_once):
+    b = admin_browser_once
+    b.visit('/sys/useradmin/')
+    assert b.path == '/sys/sudo/', (
+        'when viewing sysadmin-only pages for the first time, '
+        'the browser should be redirected to the sudo mode page'
+    )
+
+    b.fill_form({
+        'password': ADMIN_PASSWORD,
+    })
+    b.submit_by_input_name('password')
+    assert b.path == '/sys/useradmin/', (
+        'after entering password, '
+        'the browser should be redirected back to the previous page'
+    )
+
+    b.visit('/sys/groupadmin/')
+    assert b.path == '/sys/groupadmin/', (
+        'once the admin enters the password, '
+        'he would not be asked again within a certain time'
+    )