Add a :iid param to the IssuableFinder
As suggested in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6825#note_16847133, we should support finding an issuable given an :iid
param. This will move the args[:search] = "#{Issue.reference_prefix}#{args.delete(:iid)}" if args.key?(:iid)
hack down to the finder.