Handle platforms changing hosts

[IanMayo]

We recognise that some platforms have host platforms (such as a helicopter belonging to a ship).

But, we don't currently handle that changing. So - the helicopter may be assigned to one ship for a year, then move to another one.

We should handle this.

[IanMayo]

Suspect we need a HostedBy table:

• subject: (pk) platforms (fk)
• host: (pk) platforms (fk)
• hostedFrom: date (opt)
• hostedTo: date (opt)