Change /config response format, group data per coin type

[gz-c]

The current /config response data has fields like:

{
    "btc_enabled": true,
    "eth_enabled": false,
    "btc_confirmations_required": 1,
    "eth_confirmations_required": 5,
    "sky_btc_exchange_rate": "123.000000",
    "sky_eth_exchange_rate": "30.000000",
 }

Change this to group coin type-specific fields like so:

{
    "deposits": {
        "btc": {
            "enabled": true,
            "confirmations_required": 1,
            "exchange_rate": "123.000000"
        },
        "eth": {
            "enabled": false,
            "confirmations_required": 5,
            "exchange_rate": "30.000000"
        }
    }
}

This requires changes to:

• teller/http.go config handler
• README.md config API description
• Frontend config loading and handling