[RFC] Highlight historic donors

[beta-ziliani]

After #751 I have the impression we do want to highlight the companies that donated a significant amount of money even after they stop doing it. Like a "hey see how much funding these companies were interested in providing to the project".

[straight-shoota]

FTR: #751 was just fixing a bug in order to apply the current sponsorship conditions.

In general, I think we should stick to the simple rule to only give noteworthy mentions to sponsors if they are currently paying. We want to incentivise contributions after all 😏

The sponsors page lists historical data, so the amount of funding is still visible. It's not on the top of the list though (which I think makes sense, that should be reserved for current sponsors).

Perhaps we could have some perks stick around, though, even if they stopped sponsoring? I think it's hard to make any clear rules about this, though.