Bulk account reassignment

This is a new version of the earlier PR that went through a couple iterations on the NU fork. The commits are new but the diff should be virtually the same (one change: I had only added the "Move Transactions" link to `_sidenav_billing.html.haml` for the `c2po` engine, but it's in the standard partial here as well; it also wasn't localized before).
Sorry, I got this PR confused with the one in nu, which is bulk approval of users to products.