Sell/Discard slave improvements.

- Restore relative reaction text that was being discarded.
- Discard slave now discards the slave instead of redirecting to Sell Slave.  Bugged slaves can now be properly discarded again, and will reimburse the player half the calculated cost, or the minimum slave cost if the cost could not be calculated.
- Use semantic CSS class names in Sell Slave.
- Fix some missing interpolation.
- Refactor slave buyers a little bit to reduce bloat.
- A few other minor fixes.