PT-1408: Added logic to is signable to ensure items passed the signature threshold remain signable even after their deadline has passed.
Updated validation expiration.

  All validation links are valid at the time they're generated plus grace
  period.

	modified:   receive_new_signatures.inc
Updated signatures_queue settings.

  Start with 2 day grace period.

	modified:   signatures_queue.install
Updated help text.
PT-1414 - Fixed "Process Signatures" workflow doesn't check for username duplication when creating new users.
PT-1417 - Overrode legacy signature validation logic when simplified signing is enabled.
PT-1416 Added a check to see if a petition is valid before processing it.

	modified:   projects/petitions/modules/custom/petitions_api/signatures_queue/includes/process_signatures.inc