add option to suppress platform admin temporarily

so that platform admins (us) can view pages as regular users do easily.
Simply adds a flag in the session cookie that overrides the actual
platform admin flag on the user model if set. This way it's safe, since
this only downgrades existing functionality, so if someone managed to
alter it they could only get less permissions, not more.

You can change this value from the user profile page if either:

* you're a platform admin
* the flag is set (to any value) on the cookie.

This slightly weird check means that we don't check the underlying
`user._platform_admin` flag anywhere in the code, even when toggling
the suppression.

tests/app/models/test_user.py

@@ -9,7 +9,8 @@ def test_user(app_):
                  'email_address': 'test@user.gov.uk',
                  'mobile_number': '+4412341234',
                  'state': 'pending',
-                 'failed_login_count': 0
+                 'failed_login_count': 0,
+                 'platform_admin': False,
                  }
     user = User(user_data)
 
@@ -40,3 +41,21 @@ def test_activate_user(app_, api_user_pending, mock_activate_user):
 def test_activate_user_already_active(app_, api_user_active, mock_activate_user):
     assert User(api_user_active).activate() == User(api_user_active)
     assert mock_activate_user.called is False
+
+
+@pytest.mark.parametrize('is_platform_admin, value_in_session, expected_result', [
+    (True, True, False),
+    (True, False, True),
+    (True, None, True),
+    (False, True, False),
+    (False, False, False),
+    (False, None, False),
+])
+def test_platform_admin_checks_flag_set_in_session(client, mocker, is_platform_admin, value_in_session, expected_result):
+    session_dict = {}
+    if value_in_session is not None:
+        session_dict['suppress_platform_admin'] = value_in_session
+
+    mocker.patch.dict('app.models.user.session', values=session_dict, clear=True)
+
+    assert User({'platform_admin': is_platform_admin}).platform_admin == expected_result