Support Manager: Add the ability to disable the Knowledge Base

Add the ability to disable the knowledge base. This should remove any client area nav links to the knowledge base, and prevent any knowledge base articles that may exist from loading.

A setting for this could be added under Support > Knowledge Base, by use of a gear/cog icon in the upper right of the widget. Or, by adding a Manage button for the plugin under Settings > Company > Plugins > Support Manager: Manage