Allow users to change label used for Contribution Amounts in online contribution page w/o switching to a price set

Online contribution pages which use 'quick config' price sets have a hard-coded label for the contribution amounts (radio button) field. The label is stored in the price_field table but not editable in the UI unless the user switches to using a 'regular price set' for the form. For Event Online Registration - we expose the price_field.label (default 'Event Fees') and the Fees tab, making it easy for the user to modify it. We should do the same for online contribution pages when in quick config mode.

1. Add a user edit able field ("Contribution Amounts Label") to the "Amounts" tab form below the Fixed Contribution Amounts fieldset legend and above the table.
2. Update the associated quick config price field label when this form is saved.