support a media/ dir under the instance dir

this is for miscellaneous files that should be served directly rather
than being a page route

tests/test_factory.py

@@ -21,3 +21,9 @@ def test_title_override():
     response = client.get('/no-title')
     assert response.status_code == 200
     assert b'<title>suou.net</title>' in response.data
+
+
+def test_media_file_access(client):
+    response = client.get('/media/favicon.png')
+    assert response.status_code == 200
+    assert response.headers['content-type'] == 'image/png'