Allow transactions rewarding a slash to be stakeable

[Gnappuraz]

Is your feature request related to a problem? Please describe It would be nice to be able to use a TX_PAYVOTESLASH transaction to stake without having to create another transaction.

Describe the solution you'd like Similarly to https://github.com/dtr-org/unit-e/pull/687 we should allow transactions of type TX_PAYVOTESLASH to be stakeable.

Describe alternatives you've considered No alternatives were considered.