Minor admin Default display settings error

Forces the Default display as required and moves the entity based config into the entity section. Note there are good arguments for making all settings global or entity based, simply moving this into the section that the diff long text field currently checks.

This currently does not effect anything other than potentially displaying the wrong existing value in the settings page unless you select the "- None -" option in Diff 3.x-1.1. Doing this will break the marked down / Standard links when doing comparisons.