Property: GA deposits are eventually refunded

[WhatisRT]

For all proposal deposits d currently in a block, there exists an epoch e where if it contains a block, the deposit for d is not in the deposit pot anymore.

The proof goes something like this: Every proposal deposit has a corresponding GovActionState, which contains an expiresIn field. We pick that value for e. Since there was a block in e, we did a step with EPOCH into e, and if the GA was still in the state before EPOCH, it will be removed by it.