Add histogram visualisation for debates

It was requested to enable the histogram visualisation for debates. This was not enabled because the number of debates is usually very high, but it would be ok to use a size limit of 10-20 results. This would mostly be to get a sense of the most relevant debates for close reading given a keyword.

It's easy to enable the visualisation on debate IDs, but they are not very user-friendly. Perhaps we could think of using the ID fields for parties, speakers, and debates in a smarter way with the histogram visualisation.