Fix sell form to use fixed BTC/MSC rate

This fixes the incorrect handling of the exchange rate in placing a sell order. I noticed this when I tried to sell with a fixed rate of 0.15 BTC/MSC and got this:
https://masterchest.info/lookuptx.aspx?txid=b944f404b23e94cae961df161462305997d3fbb50ce67f3ad0b45a562531311b

Modified the form to use the fix rated of BTC/MSC (not messing with units such as millibits) which is how it should be handled. Also allows for easy comparison to centralized exchanges such as masterxchange.

Testing:
https://masterchest.info/lookuptx.aspx?txid=5560e74631e2eb6f38a3b17b46c0265d213cc8bd4ffc4397d4b4a1e8702985f5