Auto close margin position

**Is your feature request related to a problem? Please describe.**
Closing margin position does need to remove BTS from the margin position to the market and selling

**Describe the solution you'd like**
Option to close a margin position, when DEBT is bigger than BALANCE
Offer the amount of BTS to the market to cover the difference between DEBT and BALANCE and auto close the margin position afterwards.

*A slider for setting MSSR:
**Selling speed**
2________10%
low______high

**Additional context**
The opposite of the settle function.

Start by locating the existing margin-position closing and settle-function flows in bitshares-ui, then trace how balances, debt, and market orders are submitted. Define the expected behavior for selling the debt-balance difference, MSSR selection, and automatic closure, and add coverage for the completed flow.

Written by the indexing model from the issue text.