Include request data in CSV downloads

For CSV downloads, we could give an option to let the first line to list metadata about the query, filters, and so on. E.g.

```csv
### query text: "democracy", search_fields: [Speech, Debate title], filters: [Date: 01-01-1980:31-12-2000, Chamber: "House of Commons"]
Date;Debate title;Speaker;Speech
"1950-01-01";"blablal";"John Doe";"blablabla"
....
```

This information can be more complete and human-readable than information included in the filename.

The first line being a comment is fine for spreadsheets, and csv readers for python, R, etc. generally have the option to ignore the first n lines, but it is obviously not preferred for all users, so it should be optional.