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

bridges/bin/millau/node/src/chain_spec.rs

@@ -155,7 +155,10 @@ fn testnet_genesis(
 		pallet_grandpa: Some(GrandpaConfig {
 			authorities: Vec::new(),
 		}),
-		pallet_substrate_bridge: load_rialto_bridge_config(),
+		pallet_substrate_bridge: Some(BridgeRialtoConfig {
+			// We'll initialize the pallet with a dispatchable instead.
+			init_data: None,
+		}),
 		pallet_sudo: Some(SudoConfig { key: root_key }),
 		pallet_session: Some(SessionConfig {
 			keys: initial_authorities
@@ -165,12 +168,3 @@ fn testnet_genesis(
 		}),
 	}
 }
-
-fn load_rialto_bridge_config() -> Option<BridgeRialtoConfig> {
-	Some(BridgeRialtoConfig {
-		initial_header: Some(millau_runtime::rialto::initial_header()),
-		initial_authority_list: millau_runtime::rialto::initial_authority_set().authorities,
-		initial_set_id: millau_runtime::rialto::initial_authority_set().set_id,
-		first_scheduled_change: None,
-	})
-}