adding a variable to enable node_view Panel page, needed by all content types using the Full Page Override setting in Panelizer

Ready: Yes

- Adding a variable setting to imagex to have the node_view Panel page enabled out of the box. This is necessary for feature modules using the 'Full Page Override' for Panelizer. This way individual features won't need to include this variable with strongarm and we're avoiding these features to override each other's settings.