Make downloads table complete

At the moment, the downloads table shows all downloads made through the backend. These are:

- All downloads of search results (either immediate or with an email notification when it's done)
- Full data downloads of visualisations

However, users can also download visualisation data (not the "full data" option, just what is displayed on screen). These tables are assembled by the frontend, and therefore not stored in the downloads table in the backend. (Storing downloads was also not deemed to be as useful, since they are created quickly.)

It may be confusing to users that it works like this: why does the download table not show _all_ csv downloads? Perhaps we ought to include these downloads as well.

As for how to do this:

The quickest to implement would be to add a PUT view for the downloads - essentially the frontend just informs the backend "i made a download with these parameters and this was the csv data". (It is a bit weird in the API though, to have a view where you can store CSVs rather than request them.)

An alternative would be to have these CSVs also assembled by the backend, rather than the frontend. This is more involved but can result in a more cohesive system for all downloads. It would also enable offering download options (such as #963).