make directory redirects absolute paths

I think this is always the right choice, since we're rewriting the full
input path

incorporealcms/pages.py

@@ -27,7 +27,7 @@ def display_page(path):
     except PermissionError:
         abort(400)
     except IsADirectoryError:
-        return redirect(f'{path}/', code=301)
+        return redirect(f'/{path}/', code=301)
     except FileNotFoundError:
         abort(404)