AC Dealer changes

Includes all the changed logic to match AnthroCon's dealer system.

*Includes schema changes.*
New column: table_extras in Group table. This is a MultiChoice column.

*Includes configuration changes.*
New configuration under [enums]:
```
[[table_extra]]
power_table = "Powered Table"
wall_table = "Wall/Endcap Table"
```
A better way to express this would be

``if POWER_TABLE in self.table_extras_ints:``

A still better way would be to replace this entire block with:

```python
for extra, amount in TABLE_EXTRA_PRICES.items():
    if extra in self.table_extras_ints:
        total += amount
```

because this way there's no special-casing and you could add a new option/price without having to update this code.