Select user ids from reports using pluck

[AntonKhorev]

#4990 added the Reporter Users column. It uses reports to sort the users, but actually all it needs to get from the db are user ids. The existing version with map fetches the entire report by doing SELECT "reports".*. Here I'm replacing it with pluck that does SELECT "reports"."user_id".

[tomhughes]

It should also mean that the uniq becomes part of the database query I think. Looks good to me in any case!