Unique Usernames in Identity Pallet (#2651)

This PR allows _username authorities_ to issue unique usernames that
correspond with an account. It also provides two-way lookup, that is
from `AccountId` to a single, "primary" `Username` (alongside
`Registration`) and multiple unique `Username`s to an `AccountId`.

Key features:

- Username Authorities added (and removed) via privileged origin.
- Authorities have a `suffix` and an `allocation`. They can grant up to
`allocation` usernames. Their `suffix` will be appended to the usernames
that they issue. A suffix may be up to 7 characters long.
- Users can ask an authority to grant them a username. This will take
the form `myusername.suffix`. The entire name (including suffix) must be
less than or equal to 32 alphanumeric characters.
- Users can approve a username for themselves in one of two ways (that
is, authorities cannot grant them arbitrarily):
- Pre-sign the entire username (including suffix) with a secret key that
corresponds to their `AccountId` (for keyed accounts, obviously); or
- Accept the username after it has been granted by an authority (it will
be queued until accepted) (for non-keyed accounts like pure proxies or
multisigs).
- The system does not require any funds or deposits. Users without an
identity will be given a default one (presumably all fields set to
`None`). If they update this info, they will need to place the normal
storage deposit.
- If a user does not have any username, their first one will be set as
`Primary`, and their `AccountId` will map to that one. If they get
subsequent usernames, they can choose which one to be their primary via
`set_primary_username`.
- There are some state cleanup functions to remove expired usernames
that have not been accepted and dangling usernames whose owners have
called `clear_identity`.

TODO:

- [x] Add migration to runtimes
- [x] Probably do off-chain migration into People Chain genesis
- [x] Address a few TODO questions in code (please review)

---------

Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>
Co-authored-by: Gonçalo Pestana <g6pestana@gmail.com>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Dónal Murray <donal.murray@parity.io>

substrate/frame/alliance/src/mock.rs

@@ -19,7 +19,10 @@
 
 pub use sp_core::H256;
 use sp_runtime::traits::Hash;
-pub use sp_runtime::{traits::BlakeTwo256, BuildStorage};
+pub use sp_runtime::{
+	traits::{BlakeTwo256, IdentifyAccount, IdentityLookup, Lazy, Verify},
+	BuildStorage, MultiSignature,
+};
 use sp_std::convert::{TryFrom, TryInto};
 
 pub use frame_support::{
@@ -101,6 +104,7 @@ parameter_types! {
 	pub const MaxSubAccounts: u32 = 2;
 	pub const MaxAdditionalFields: u32 = 2;
 	pub const MaxRegistrars: u32 = 20;
+	pub const PendingUsernameExpiration: u64 = 100;
 }
 ord_parameter_types! {
 	pub const One: u64 = 1;
@@ -124,9 +128,34 @@ impl pallet_identity::Config for Test {
 	type Slashed = ();
 	type RegistrarOrigin = EnsureOneOrRoot;
 	type ForceOrigin = EnsureTwoOrRoot;
+	type OffchainSignature = AccountU64;
+	type SigningPublicKey = AccountU64;
+	type UsernameAuthorityOrigin = EnsureOneOrRoot;
+	type PendingUsernameExpiration = PendingUsernameExpiration;
+	type MaxSuffixLength = ConstU32<7>;
+	type MaxUsernameLength = ConstU32<32>;
 	type WeightInfo = ();
 }
 
+#[derive(Clone, Debug, Encode, Decode, PartialEq, Eq, TypeInfo)]
+pub struct AccountU64(u64);
+impl IdentifyAccount for AccountU64 {
+	type AccountId = u64;
+	fn into_account(self) -> u64 {
+		0u64
+	}
+}
+impl Verify for AccountU64 {
+	type Signer = AccountU64;
+	fn verify<L: Lazy<[u8]>>(
+		&self,
+		_msg: L,
+		_signer: &<Self::Signer as IdentifyAccount>::AccountId,
+	) -> bool {
+		false
+	}
+}
+
 pub struct AllianceIdentityVerifier;
 impl IdentityVerifier<AccountId> for AllianceIdentityVerifier {
 	fn has_required_identities(who: &AccountId) -> bool {
@@ -135,7 +164,7 @@ impl IdentityVerifier<AccountId> for AllianceIdentityVerifier {
 
 	fn has_good_judgement(who: &AccountId) -> bool {
 		if let Some(judgements) =
-			Identity::identity(who).map(|registration| registration.judgements)
+			Identity::identity(who).map(|(registration, _)| registration.judgements)
 		{
 			judgements
 				.iter()