[fix bug 1025701] /tour should have different UITour PageID than /firstrun

The Australis tour specifies a page ID which is used with [Telemetry](https://wiki.mozilla.org/Telemetry).

Currently the firstrun tour is available at both /firstrun and /tour. The Firefox folks would like to specify a different ID for Telemetry based on which URL is used to access the firstrun tour.

This PR changes from using hard-coded string ID's to using a data-attribute in the template (which also has the added bonus of removing some duplicate JS code from /whatsnew and /firstrun).