clean up amount/fee semantics in exchanges table

[chadwhitacre]

The relationship between the amount and fee columns in the exchanges table is counterintuitive. Reticketing from #1118 (comment):

I'd like to to continue the discussion about what would be the principle of least surprise regarding the exchanges table.

The least surprising would be if both payins and payouts behaved the same. I'd like to be able to do either one of

• balance = balance + amount - fee
• balance = balance + amount

not depending on the sign of the amount. It does not really matter which one. The first variant behaves like when we are taking the fee of the balance. The second variant behaves like the user pays the fee before the money reaches gittip or after it has left. Both are fine choices.

Want to back this issue? Post a bounty on it! We accept bounties via Bountysource.

[chadwhitacre]

#3653 will obviate this.

[chadwhitacre]

So much for #3653. Still, this ticket should be informed by that one.

[chadwhitacre]

Closing in light of our decision to shut down Gratipay.

Thank you all for a great run, and I'm sorry it didn't work out! 😞 💃