CRM-12233 Label for newly create quick config price fields for membershi...

...p block should be Membership, not Membership Amount. Leaving the price_field.name as membership_amount since that's clearer as a machine name.

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* CRM-12233: Online membership signup - contribution amounts field should not be required + various notices
  http://issues.civicrm.org/jira/browse/CRM-12233