Transaction sort by amount doesn't take inflow/outflow into account

[sbine]

When sorting by transaction amount, sign (positive/negative, which is represented in bdgt by the inflow boolean on each transaction) should be taken into account. Currently negative outflow numbers are mixed in with positive inflow numbers–see the screenshot below, where the correct order would be $1,000, $100, -$59.99, -$90.51, -$155.22, -$200.

• Sorting is done in TransactionsTable.vue (by vue-tables-2).
• To work on this task you'll need to know how to build the frontend assets.

[vclayton]

I'd like to take a crack at this.