add ability to suspend accounts

[chadwhitacre]

I don't want to pull money in from stolen credit cards (#329).

[chadwhitacre]

I applied the second version of the schema update (from 4d3a249) to the production db.

[chadwhitacre]

This is only for payin. Reticketed suspending payouts as #353.

[ironchefpython]

Might I suggest an additional state to supplement suspension? Perhaps call it, "delayed".

The delayed state would allow cards to be charged, but the funds would remain in escrow, (temporarily) unable to be withdrawn by the giftee. And adding a new credit card to an account automatically triggers this state for (hypothetically) 4 weeks. And when the account transitions from "delayed" to active, all escrowed funds would be available for withdrawal.

This makes profiting from stolen credit cards significantly more difficult, will facilitate chargebacks without having to "claw-back" funds from bank accounts, and has a minimal impact of long-term gittips.

[chadwhitacre]

@ironchefpython I think the best way forward is to implement a whitelist: only explicitly approved accounts can put money into the system. My goal is to build a reviewing facility (#354, #355) and to modify payday accordingly (#356).