Allow Substrate Pallet to be Initialized via Dipatchable (#481)

* Add dispatchable for intializing pallet

* Add Polkadot JS types for Substrate bridge pallet

* Ensure Root is the only one that can initialize the pallet

* Add some tests

* Pack initialization data into struct

* Only allow pallet to be initialized once

* Use new initialization config in nodes

* Rename ScheduledChange in Ethereum pallet

We're renaming it to prevent clashes with Substrate bridge pallet type
of the same name. This is relevant when importing types to Polkadot JS
Apps.

* Move all Polkadot JS types into one file

* Appease Clippy