Add ::gaTrackingId:: footer token

[nickdos]

Similar to #16, it would be good to be able specify a testing GA tracking ID so our GA stats are not polluted by hits from our test servers (particularly automated tests).

Requires the ::gaTrackingId:: var to be added to https://github.com/AtlasOfLivingAustralia/commonui-bs3/blob/master/footer.html#L154 and then the var to be substituted via a grailsApplication.config variable.

Ansible templates and inventories will need to be updated for all front-end apps (needs child issue).

[matthewandrews]

Worth considering Google Tag Manager instead - a single code snippet in all sites, then we manage centrally which sites are test and which are prod: https://developers.google.com/tag-manager