Update inflation model to model with parachains

The ideal staking rate in the inflation model of Polkadot is still hardcoded to be 75% [here](https://github.com/paritytech/polkadot/blob/master/runtime/polkadot/src/lib.rs#L523). On Kusama there is already a more advanced mechanism that scales with the number of parachains and it is probably time to update Polkadot to that model.