Add active flag to RpcPerson model

[jennifer-richards]

The RpcPerson model needs an active flag that can be set to False to represent people who should no longer have work assigned to them.

Places where assignments can be made will need to check and respect this flag.

In addition to people who have changed roles / left the RPC, this will apply to people who have died. Care should be taken with language to be respectful of the latter possibility, especially where it may affect the UI.