Feature: chained transfer / 3rd party approved transfer

Inspired by https://github.com/bitshares/bitshares-ui/issues/462, we can have "UI for approving transfers for assets that require it", via proposal, like another feature proposed in https://github.com/bitshares/bitshares-ui/issues/866.

Assume A wants to transfer some asset X to another user C, and B is the issuer of asset X which requires issuer's approval for transferring, steps to do an approved transfer:
1. A create a proposal which contains 2 operations:
* A transfer amount M of X to B
* B transfer amount M of X to C
2. A approve the proposal
3. B approve the proposal

The 2 transfers will either both pass or both fail (atomic).

On the transfer page, we can have an option like:
- [ ] transfer via _____

We can pre-fill the text box with known agents, or the asset issuer, or a value saved by the user, or show a list, etc.

Note: since funds from A is not locked, this feature is not actually an escrow.

Start with the transfer page and the proposal approval flow described in the issue, then review the linked BitShares UI issues for prior context. Done means users can initiate the two-transfer proposal, identify or select the intermediary, and have both transfers succeed or fail atomically after the required approvals.

Written by the indexing model from the issue text.