Added a toggle for placing limit orders - post-only.

When placing a limit order, you can set the toggle Post-only. With this toggle it is guaranteed that when executed, the order will work as a maker or will be cancelled by the system.

This toggle will be useful for those who have more favorable terms on maker fees and want to guarantee execution of an order with the maker role during trading on the market.

As a reminder, favorable terms for makers can be obtained with WBT holding. You can read more here: https://whitebit.com/wbt.