Add event handlers for google analytics

[nsolomon-nycc]

• [x] Dropdowns
• [x] Page
• [x] Downloads
• [ ] ~Graph clicks~
• [ ] ~Graph hovers~
• [ ] ~Map clicks~
  • [ ] ~Zoom/pan~
  • [ ] ~Popups~

[nsolomon-nycc]

widget clicks are trickier to track than I thought they would be, so I think we should plan to implement that a bit further down the line.

For now, I'm going to migrate adding them to a separate issue.