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/rialto/node/src/chain_spec.rs

@@ -186,9 +186,6 @@ fn load_kovan_bridge_config() -> Option<BridgeKovanConfig> {
 
 fn load_millau_bridge_config() -> Option<BridgeMillauConfig> {
 	Some(BridgeMillauConfig {
-		initial_header: Some(rialto_runtime::millau::initial_header()),
-		initial_authority_list: rialto_runtime::millau::initial_authority_set().authorities,
-		initial_set_id: rialto_runtime::millau::initial_authority_set().set_id,
-		first_scheduled_change: None,
+		init_data: Some(rialto_runtime::millau::init_data()),
 	})
 }

bridges/bin/rialto/runtime/src/millau.rs

@@ -18,11 +18,23 @@
 
 use bp_rialto::Header;
 use hex_literal::hex;
-use pallet_substrate_bridge::AuthoritySet;
+use pallet_substrate_bridge::{AuthoritySet, InitializationData};
 use sp_core::crypto::Public;
 use sp_finality_grandpa::AuthorityId;
 use sp_std::vec;
 
+/// Information about where the bridge palelt should start syncing from. This includes things like
+/// the initial header and the initial authorities of the briged chain.
+pub fn init_data() -> InitializationData<Header> {
+	let authority_set = initial_authority_set();
+	InitializationData {
+		header: initial_header(),
+		authority_list: authority_set.authorities,
+		set_id: authority_set.set_id,
+		scheduled_change: None,
+	}
+}
+
 /// The first header known to the pallet.
 ///
 /// Note that this does not need to be the genesis header of the Millau