Hide template folder permission editing for platform admin users

Platform admin users can access all template folders, so the folder
permissions form always displays everything as checked for them,
which makes it look like the form isn't actually working. We could
do the check based on folder data, but the field still wouldn't
have any effect on permissions. So instead, we hide it completely
for platform admin users.

Submitting the form will remove any folder permissions from the DB
for the platform admin user (which can still be created by changing
permissions on the template folder 'Manage' page), but that's only
relevant if a user stops being a platform admin but keeps their
Notify services.

tests/app/main/views/test_manage_users.py

@@ -17,6 +17,7 @@ from tests.conftest import (
     active_user_view_permissions,
     active_user_with_permissions,
     normalize_spaces,
+    platform_admin_user,
     sample_uuid,
 )
 
@@ -500,6 +501,54 @@ def test_edit_user_folder_permissions(
     )
 
 
+def test_cant_edit_user_folder_permissions_for_platform_admin_users(
+    client_request,
+    mocker,
+    service_one,
+    mock_get_users_by_service,
+    mock_get_invites_for_service,
+    mock_set_user_permissions,
+    mock_get_template_folders,
+    fake_uuid,
+):
+    service_one['permissions'] = ['edit_folder_permissions']
+    mocker.patch(
+        'app.user_api_client.get_user', return_value=platform_admin_user(fake_uuid)
+    )
+    mock_get_template_folders.return_value = [
+        {'id': 'folder-id-1', 'name': 'folder_one', 'parent_id': None, 'users_with_permission': []},
+        {'id': 'folder-id-2', 'name': 'folder_one', 'parent_id': None, 'users_with_permission': []},
+        {'id': 'folder-id-3', 'name': 'folder_one', 'parent_id': 'folder-id-1', 'users_with_permission': []},
+    ]
+    page = client_request.get(
+        'main.edit_user_permissions',
+        service_id=SERVICE_ONE_ID,
+        user_id=fake_uuid,
+    )
+    assert page.select_one('main p').text == 'foo'
+    assert page.select('') is False
+    client_request.post(
+        'main.edit_user_permissions',
+        service_id=SERVICE_ONE_ID,
+        user_id=fake_uuid,
+        _data=dict(
+            folder_permissions=['folder-id-1', 'folder-id-3']
+        ),
+        _expected_status=302,
+        _expected_redirect=url_for(
+            'main.manage_users',
+            service_id=SERVICE_ONE_ID,
+            _external=True,
+        ),
+    )
+    mock_set_user_permissions.assert_called_with(
+        fake_uuid,
+        SERVICE_ONE_ID,
+        permissions=set(),
+        folder_permissions=['folder-id-1', 'folder-id-3']
+    )
+
+
 def test_cant_edit_non_member_user_permissions(
     client_request,
     mocker,