[Sponsors] Change display of Sponsors tally

On https://scipost.org/sponsors/, admin has access to a tally of sponsorships per year. At the moment, it counts Sponsors only on their latest sponsoring year. It would be useful to have the total number of Sponsors in each of the time frames defined, i.e. the number of organisations with at least one subsidy within that period: Current, Last year, 2 years ago, Older, Total, Never.