GA implementation needs updating

[rmccar]

The GA implementation needs updating. The gtag function isn't able to be called because it is initialised from within another function and its a function that needs to be setup on the service anyway as part of the GA setup so we shouldn't need to worry about initialising it. It being not initialised by us would mean that the service gets an error in the console as to why analytics wont work in this case which would be helpful.
Also as part of this we should prefix our events with "ons_" to avoid using reserved word events when they are sent to GA.