Export tags and document link in .csv

[BeritJanssen]

Close #1195, also updated the download manual, which still described the situation prior to the download module.

[lukavdplas]

Without going into the code implementation, I have a few general comments:

• Did you see Support extra csv columns for search results download #1195 (comment) ? If so, is there a reason you went in a different direction?
• The download component includes a "additional columns" section to include non-field columns. (Currently hidden because query-in-context downloads don't work in the backend, and that was the only option. So I can imagine you may have missed this.) I think this would be the appropriate place to include tags and the url, rather than in the field dropdown. (This field dropdown is like, 5 cm below another field dropdown that does not include those options.)
• The manual update overlaps with more manual #1811 which also updates the download manual to the current situation. No changes requested here, just pointing it out.