Remove "All time" link from Universal Analytics

* Was expensive
* This is just a removal of the link; appending `period=all-time` to a
  Universal Analytics URL query will still, at present allow you to *try* to reach it.
 
We *could* 400 Bad Request it, but that would involve *adding* code; UA/All time requires no special code over and above what exists for other areas, so there's nothing to remove as such.
I wonder if this logic belongs in the controller and/or in a method on `TimePeriod`? It keeps the knowledge of the slugs to fewer places. Putting it in the controller would also reduce the complexity of the view.