Instrument with GA (bug 903050)

This adds GA analytics based on fireplace's tracking js with UA parts removed - we can add that back in once it stabilises. 

Other parts to note:

* npm install will install the settings_local.js for js overrides
* Overriding js settings can be done via python settings see the added docs 

To test analytics inclusion locally you'll need to add a line to `webpay/settings/local.py` like so:


```python
base.JS_SETTINGS['tracking_enabled'] = True
```

Tracking is just per-page view, events are a separate bug to follow.