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 will still, at present allow you to *try* to reach it.
Move knowledge of slugs away from views

* Add options to `View::Hits::TimePeriod.all` to allow exclusion of all-time period
* Also tidy up some related specs that were incorrectly nested under `describe '.all'`
Merge pull request #474 from alphagov/remove-universal-all-time-link

Remove "All time" link from Universal Analytics