[BW]: [NIP]:

- Defined remove_proposal_operation
- Fixed authority checks in create_proposal, update_proposal_votes_operation
- url passed to create_proposal must be valid permlink.
- defined length limits for passed proposal_ids and subject in SPS operations
- proposal_ids (in update_proposal_votes_operation and remove_proposal_operation) are stored in flat_set, to avoid redundants. Also removed unneeded copy from wallet.