Standardize on "gross" for donation amount

Recurring donations used "amount", but we want to reuse the currency conversion
logic, so...

Also fixes a currency import error on subscr_signup.  This is a very old bug.
The impact is that civicrm_contribution_recur.currency is null for a random
portion of paypal signups.

Change-Id: I25a6409c51a6b277fa65edd9c5934ac2087955f0