TestnetSwap - non-custodial cross-chain atomic swaps in the browser (Monero cross-curve, testnet/stagenet), plus TestnetHub to learn the testnets

I've been building a suite of testnet tools for a while (a faucet, a wallet, an explorer). Two new pieces just went live and they go together: TestnetSwap, non-custodial cross-chain atomic swaps that run entirely in the browser, and TestnetHub, a place to actually learn how the testnets work. TestnetSwap swaps tLTC <-> tBTC and Monero (tXMR/sXMR) into tBTC or tLTC (the "t" prefix is testnet and…