Schema changes for soft credit / PCP implementation

Create new join table: civicrm_contribution_soft. Table columns are:

• contribution_id - FK to contribution.id
• contact.id - FK to contact.id (for PCP contributions, this will be
  the pcp.contact_id value, for back-office soft-credits, just the
  contact being credited)
• amount - in order to support partial or split soft credits, we'll
  store the amount credited to this contact here - for PCP it will be
  equal to contribution.total_amount
• pcp_id - FK to pcp.id (null for back-office soft credits)
• pcp_display_in_role (pcp use only for now)
• pcp_roll_nickname (pcp use only for now)
• personal_note (pcp use only for now)