Provide an option to enable/disable submenu tree per content type

Although default Submenu Tree options can be provided per content type, there is currently no way to completely enable or disable Submenu Tree for a desired content type. This can cause a number of small problems including having no way to know when to add/delete Submenu Tree database records and not being able to allow sub/sibling options to be customized without giving users menu-editing permissions.

Historically, Submenu Tree 7.x-1.x deleted all Submenu Tree database records for a node whenever the node did not have sub/sibling content enabled. Unfortunately, we can no longer do this while allowing administrator's to provide a default value for enabling sub/sibling (this was the cause of #941596: Default settings options per content type.

7.x-1.x also only presented Submenu Tree options on the node edit form whenever menu options were present. This prevents the ability to give a website editor the power to change a node's sub/sibling content options while restricting them from editing the node's menu.

After a number of tests, I believe that the best way to solve these issues (and a few other potential problems that are not worth mentioning) is by adding an option to the Submenu Tree form on the Content Types page which would allow an administrator to enable/disable Submenu Tree per content type.