sponsors.js should probably be deduplicated

[jducoeur]

docs/js/sponsors.js drives the list of financial contributors on the sponsors page. This is driven mainly by loading the list of sponsors from OpenCollective.

We recently fixed a bug that was omitting sponsors (one of the Gold-tier sponsors pointed it out), but that revealed a different bug: we're now showing duplicate entries for some organizations and people. (Including myself.)

I suspect that what's going on is that the OC GraphQL query is returning every "membership" -- each distinct tuple of (member, tier) -- rather than the members themselves. So if an organization has changed its subscription, or someone has gone from ad-hoc to recurring contributor, they have multiple entries and we show it multiple times.

The current situation isn't tragic, but I'd recommend that we deduplicate this list, and only show the highest tier that each member occupies.