Fixup for https://github.com/paritytech/polkadot-sdk/pull/2587 to make
the `core-fellowship` crate work with swapped members.

Adds a `MemberSwappedHandler` to the `ranked-collective` pallet that are
implemented by `core-fellowship+salary`.
There is are exhaustive tests
[here](https://github.com/paritytech/polkadot-sdk/blob/72aa7ac1/substrate/frame/core-fellowship/src/tests/integration.rs#L338)
and
[here](https://github.com/paritytech/polkadot-sdk/blob/ab3cdb05

/substrate/frame/salary/src/tests/integration.rs#L224)
to check that adding member `1` is equivalent to adding member `0` and
then swapping.

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>