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).
This class applies to the account `<select>` on the 2nd step of chart string reassignment, not the checkboxes on the first screen, which should be consistent with the styling on the Send Transactions screen.

(BTW I tried applying a 20px left padding to those checkboxes just now and I kind of liked the result, but it's not a huge difference.)