Remove custom 'destroy' methods on join tables

* now handled by $.ajaxPrefilter
Make object tree selectors work

* fix links and make modal-selector accept 'join-object-id'
  data parameter
* add CMS.Models.SystemSystem and CMS.Models.SystemControl
* fix 'system_systems' endpoint (from 'systems_systems')
Merge pull request #51 from reciprocity/feature/more-selector-work

Handle system/system and system/control selectors in object tree.