Add graph view settings to the workflow admin panel

[Keluri]

In addition to the hardcoded settings in trac.ini, having settings directly available on the workflow administration panel would allow for easier previewing of workflows. Also, in conjunction with #12014, this would allow users who cannot edit trac.ini to preview images in a way that suits them.

The following options should be present on the workflow administration panel:

• diagram_colors
• diagram_font
• diagram_fontsize
• diagram_size

In addition, the following options should be added to the workflow-admin section in trac.ini:

• max_diagram_size

The addition of a max_diagram_size prevents abuse of the diagram_size option that would be available on the admin page (Preventing, say, a 10^24 x 10^24 image being rendered).

[Jun Omae]

diagram_font option must not be changed by non TRAC_ADMIN users because the option is set to local path in the server.