Vesting accounts: Add clawback

[SpicyLemon]

Summary

Add clawback ability to vesting accounts.

Problem Definition

As a granter of vesting funds, I want to be able to recover (clawback) unvested coins in some circumstances so that I can more easily enforce grant restrictions.

Proposal

Make changes similar to the ones made by Algoric to add clawback abilities to vesting accounts.

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[SpicyLemon]

Related: #945

[iramiller]

We should consider using a hold module/encumbrance model for this. If we had an encumbered transfer then we could have an associated owner of the encumbrance which should be able to create a single transaction that cancels the hold and pulls the funds back if signed by the encumbrance owner.