Notes and commentable page

Actually every page (if globally enabled) could be commented. I will add
several comments to that one, just to show how they look like. Also I
will add an ordinary git note and it will be displayed separately.

This is an example note in refs/notes/commits namespace.

comment 0:

From: Sergey Matveev
Date: 2020-05-09 11:51:49Z

The very first comment here.

comment 1:

From: Sergey Matveev
Date: 2020-05-09 11:52:09Z

And yet another one.

comment 2:

From: Sergey Matveev
Date: 2023-05-05 16:37:20Z

another test comment after server upgrade