Add Google Analytics tracking

We track:
* Button clicks
* External link clicks
* Addendum download links
* Form validation errors
* File upload drag drop vs browse
* File upload timing
* File upload errors
* Modals being opened
* Which upload link was used to get to upload page
* (Virtual) pageview for second upload form stage
* Logged in status as a dimension

I'm using [this](http://www.seerinteractive.com/blog/event-tracking-naming-strategy) as a basis for our event naming.