Listing & editing of Pages based on group selection

1. This code contains listing and editing of objects only for 'Page' GSytemType objects.

2. Apart from this important points to note are as follows: 

  (a) group_set field is added in GSystem class.

  (b) In "node_edit_base.html", a block tag (named "base_save_on_click") is added inside Save-button's onclick jquery function. So that other's can add their code into that, that needs to be performed on Save-button's click.