Lost card association after merging accounts

[ironfroggy]

I merged my twitter and github accounts (same names) and no longer have a card associated, or a history I can see.

I was not charged the last time I should have to replenish my balance.

[ceboudreaux]

@whit537 Does merging ever/always clear credit card data?

[chadwhitacre]

This is a dupe of #818 but it looks like I closed that one without actually fixing it, so let's leave this open to pick up with this.

[chadwhitacre]

See #1854 for cleaning up card associations in the case where a Gittip account is closed.

[Changaco]

Closing in favor of #2630.