Improve performance of network graph

Created by: hiroponz

I think that loading network graph is as fast as that of 5.0.

Overview

1. Rendering graph partially.
2. Reduce database access to get notes.
3. Enable vertical scrolling.Thanks @giggsey.
4. Some minor bugs related to network graph is fixed.

Dragging network graph is disabled, because it conflicts with scroll.

Screenshot

fixes #3684 (closed) refs #3711