Add sponsorship information

This adds a dedicated sponsors page, and a sponsors section to the homepage. The sponsors page shows all backers and sponsors, while the homepage only shows sponsors (those who donate more than €100/month). The sponsorship data pulled from Open Collective, and all images are mirrored. This removes the need for depending on Open Collective when serving web requests, and ensures we don't end up displaying many large logos; increasing the time to load pages.