Add Google Analytics tracking

[ucamhal]

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 as a basis for our event naming.

[timdegroote]

Finished an initial round of review, reassigning to @ucamhal

[ucamhal]

Rebased onto the current cul-ds master to fix merge conflict. Commits from 619930d onwards are review follow up from your comments.

[micheleidesmith]

Any ideas when we can release this? Lihua is working on dashboard design and it would be great to start configuring GA. :)