[Proposal Improvements] Add fee paying account to `available_active_approvals`

_From @xeroc on December 13, 2015 17:38_

Let's say we have a user account `Alice` that is secured by a second factor service.
The active authorities of `Alice` requires 2 signatures, one for each
- `Bob`
- `2FA`

Now, I'd like to have `Bob` propose a transaction and only have `2FA` add his approval to have the proposal withdrawing from `Alice` be valid. That would GREATLY improve user experice and does not open up any security holes since `Bob` needs to be in the active authority any way.

The only difference is that to verify the proposal, you also need to include the proposals signature (e.g. fee paying account).

However, I think this feature request requires a hard fork.
I am open for discussion

_Copied from original issue: cryptonomex/graphene#479_

Start by locating `available_active_approvals` and read the issue's proposal-signature requirement alongside linked pull request #1368. Determine how the fee-paying account participates in approval validation and whether the hard-fork implications are resolved; done means the requested approval flow is implemented and validated against the stated Alice, Bob, and 2FA scenario.

Written by the indexing model from the issue text.