add page modified time to the template

tests/test_pages.py

@@ -1,4 +1,6 @@
 """Test page views and helper methods."""
+import re
+
 from incorporealcms.pages import page_file_resolver
 
 
@@ -36,3 +38,9 @@ def test_page_without_title_metadata(client):
     assert response.status_code == 200
     assert b'<title>incorporeal.org</title>' in response.data
     assert b'<h1>this page doesn\'t have a title!</h1>' in response.data
+
+
+def test_page_has_modified_timestamp(client):
+    response = client.get('/')
+    assert response.status_code == 200
+    assert re.search(r'Last modified: ....-..-.. ..:..:.. ...', response.data.decode()) is not None