proposal_delete_operation issues

_From @abitmore on January 9, 2016 4:3_

1. proposal_delete_operation doesn't recursively check the authorities in case when multi-sig authority involved. See https://bitsharestalk.org/index.php/topic,20861.msg271080.html#msg271080, it's a try to delete an incorrect committee proposal, but failed due to this reason:
`
10 assert_exception: Assert Exception
required_approvals->find(o.fee_paying_account) != required_approvals->end(): Provided authority is not authoritative for this proposal.
{"provided":"1.2.12376","required":["1.2.0"]}
`
2. `This operation allows the early veto of a proposed transaction. It may be used by any account which is a required authority on the proposed transaction, when that account's holder feels the proposal is ill-advised and he decides he will never approve of it and wishes to put an end to all discussion of the issue.`
Imo it's better to set a threshold equals to (total_weight - approve_threshold) for proposal_delete_operation, so the proposal will only be deleted if enough authorities agree to delete the proposal.

_Copied from original issue: cryptonomex/graphene#515_