Add advanced analytics

[robredpath]

We should use our analytics system to get a better understanding of how people use Insights. We'll want to build on this over time, but to start with:

• We want to make sure that we know what people are putting into the search boxes
• We want to know which controls people are clicking on
• We want to know how many people are downloading / printing

For searching, we might need to use Matomo's trackSiteSearch() function, as regular site search is, I think, predicated on there being query params.

For controls / printing, we probably want to use Events as they give us a good number of variables to send information back with. We can either trigger these with onClick or, if that's awkward, potentially on page load through URL parsing - although that might be tricky in its own right to understand what's just been clicked. Details to work out.