Online contribution pages should utilize the existing buildPCP method to add PCP fields

Event registration forms use a share method to add PCP form fields (CRM_PCP_BAO_PCP::buildPcp). See CRM_Event_Form_Registration_Register::buildQuickForm. However this code is pretty much replicated in CRM_Contribute_Form_Contribution_Main::buildQuickForm. Instead that class should use the shared method. Please review for any adjustments needed to the shared method to handle both forms and make sure the logic for assigning text to pcpSupporterText smarty variable handles both conditions (link_text present / link_text empty).