sp-qp-widget: add tags to new posts

[Rafi Yagi]

Users should be able to tag new posts.

Site administrators should also be able to turn this feature on and off on the back-end in the settings section. By default, this should be on.

[Rafi Yagi]

Tagging is now added to the SP QuickPost widget and the SP Post Settings widget. For now, I am not going to add a feature to turn this on or off.