Add gather_sponsors_for_petition to petition_mailer_preview
Add gather_sponsors_for_petition to petition_mailer_preview

unboxed/e-petitions,unboxed/e-petitions,unboxed/e-petitions,oskarpearson/e-petitions,joelanman/e-petitions,StatesOfJersey/e-petitions,alphagov/e-petitions,alphagov/e-petitions,ansonK/e-petitions,telekomatrix/e-petitions,unboxed/e-petitions,joelanman/e-petitions,ansonK/e-petitions,StatesOfJersey/e-petitions,oskarpearson/e-petitions,telekomatrix/e-petitions,joelanman/e-petitions,StatesOfJersey/e-petitions,alphagov/e-petitions,ansonK/e-petitions,alphagov/e-petitions,StatesOfJersey/e-petitions,oskarpearson/e-petitions,telekomatrix/e-petitions