fixes #4409 - rename the id used for the content menu

For some reason, using 'content' as the id for the Content menu
was resulting in breakage in behavior for foreman's two-pane
implementation.

Without this change, the following would occur:
- User goes to a page, clicks 'New <object>' (e.g.
  Architecture, Environment, User...)
- panel opens with form to allow user to enter
  data for the object
- User clicks submit
- At this point, the form is submitted to the server
  and the object is created; however, the form is never
  closed and the list of objects is not updated.