Closes #78: fix managed item to show buttons on multisite/test instances

This modifies the managed entity so that it populates using a relative URL instead of an absolute URL. That way the buttons will continue to appear on dev/test instances, as well as most multisite configurations.

To test this, you can look at the search display(s) in question, press "Preview" to see the buttons, then clear cache. You should see the include path change, but the buttons will still be present.