XNAT-4533, more: Added Plugin Settings page - 'Administer > Plugin Settings'...

XNAT-4533, more: Added Plugin Settings page - 'Administer > Plugin Settings' menu option will show when there's an installed plugin that has site-wide settings. The mechanism for this is clunky and could be improved if there was a way for plugins to register themselves as having site-wide settings. Split up items in 'Administer' menu for more granularity in inserting menu items. Other refactoring to support these features, including a new panel.form contentType option 'text/json' that submits JSON as text (needed for XSync plugin). (WHY!?)