Google Analytics classic data collection techniques are being
deprecated by Google. We no longer need to support classic and any new
tracking should use only Universal.

Simplify tracker object by moving for loops into a single method which
gets passed the function to call on each tracker and the arguments to
give it.

Add some protection against methods not existing on the trackers.

GOVUK.Analytics was formerly a namespace used by slimmer. This has been
removed and all apps using slimmer updated.

Usage goes from
`GOVUK.analytics = new GOVUK.Tracker(…)`
to
`GOVUK.analytics = new GOVUK.Analytics(…)`

The analytics object becomes the container for trackers.

* Remove references to classic, classic’s custom variables and classic
specific implementation.
* Refer to the new Analytics object rather than Tracker
* Update the init example.

Remove duplication of `beforeEach`