Add sponsors to the Rails Girls Summer of Code page

[svenfuchs]

Just like Paymill on /grants and /grants/rvm, we should also add all the RgSoC sponsors to /rgsoc

[anikalindtner]

@svenfuchs is this still something you'd like to see? maybe we add the partners and platinum sponsors only? (and for what year?)

[alicetragedy]

Since we're currently working on the promised "Hall of Fame" on the RGSoC site, we could definitely link to that. Possibly also add partners/platinum sponsors directly as @anikalindtner suggested — but then we would have to add them for all years, right?