PledgeBank: Implement Find Pledge Signers screen

Elements:

• Search form elements:
  o signer name: input text
  o pledge name: ajax selector
  o pledge deadline from: date selector
  o pledge deadline to: date selector
  o pledge done: checkbox
• Selector columns:
  o Name: (display_name for civicrm_pb_signer.signer_id), linked to contact record
  o Pledge: (literally: "...will (civicrm_pb_pledge.signer_pledge_desc)"), linked to pledge view page
  o Signed on: (civicrm_pb_signer.signing_date)
  o Anonymous? (civicrm_pb_signer.is_anonymous)
  o Done? (civicrm_pb_signer.is_done)
  o Find test signatures (civicrm_pb_signer.is_test)