Google Analytics event tracking

This adds event tracking to a bunch of stuff:

* subscribe to policy
* unsubscribe to policy
* edit policy
* new policy
* submit policy
* edit division
* submit division edit
* click signup
* submit signup
* share on twitter
* share on facebook

There's one TODO it would be good to clear:

Only run the event tracking script if google analytics is loaded:
https://github.com/openaustralia/publicwhip/compare/ga_event_tracking?expand=1#diff-00a3b00eb80309789c76db7d42fc1e6cR2

I'm not sure of the best way to do that and haven't found very helpful answers. @henare any thoughts?

closes #996