Crypto donations

This MR adds:

1. A half-baked, unlinked donate page at /donate/crypto. It is only configurable through Soapbox Config > Advanced, like so:

{
  ...
  "crypto_addresses": [
    {
      "address": "bc1q9cx35adpm73aq2fw40ye6ts8hfxqzjr5unwg0n",
      "ticker": "btc",
      "note": "Optional note"
    },
    {
      "address": "0xAc9aB5Fc04Dc1cB1789Af75b523Bd23C70B2D717",
      "ticker": "eth"
    },
    {
      "address": "D5zVZs6jrRakaPVGiErkQiHt9sayzm6V5D",
      "ticker": "doge"
    },
    {
      "address": "0x541a45cb212b57f41393427fb15335fc89c35851",
      "ticker": "ubq"
    },
    {
      "address": "45JDCLrjJ4bgVUSbbs2yjy9m5Mf4VLPW8fG7jw9sq5u69rXZZopQogZNeyYkMBnXpkaip4p4QwaaJNhdTotPa9g44DBCzdK",
      "ticker": "xmr"
    }
  ],
  ...
}

2. Renders user profile fields as crypto addresses when a ticker like $BTC or $DOGE is set as the title.

Future MRs will expand upon this.