Add Stellar support

I would love to see Brave integrate with the Stellar network so we can get BAT tokens directly on there instead of on Eth. There’s already a trustline on Stellar for it from apay.io, but it would be better to have Brave as an anchor issuing them directly. Compared to Eth, Stellar’s fees almost don’t exist, and since it’s a built-in SDEX, you could even, in theory, issue a payment to someone’s wallet via a path payment so they receive something other than BAT in their wallet, like Brazilian Reals, without Brave ever having to have or touch that fiat currency.