Support proper "additional percentage" functionality on back-office forms

[twomice]

Currently (and, I believe, since creation), this extension does not properly calculate additional percentage on back-office forms such as "New Contribution".

Current behavior examples:

• "Add Membership" and "Submit Credit Card Membership":
  • The "additional percentage" checkbox displays with the cryptic label ERROR: percentagepricesetfield_NFMbsQAQVpVZkamC (Hint: Is the Percentage Price Set Field extension enabled?) (see 'Issue on "Credit Card Membership"' #45)
  • The checkbox has the effect of adding exactly $1 to the total.
• "New Contribution", "Submit Credit Card Contribution", "Add Event Registration", and "Submit Credit Card Event Registration":
  • The "additional percentage" checkbox displays properly (no weird label).
  • The checkbox has the effect of adding exactly $1 to the total.

Desired behavior on all above-listed forms:

• The "additional percentage" checkbox displays properly (no weird label).
• The checkbox has the effect of adding the correctly calculated additional percentage to the total