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/bin/node/runtime/src/lib.rs

@@ -1500,6 +1500,12 @@ impl pallet_identity::Config for Runtime {
 	type Slashed = Treasury;
 	type ForceOrigin = EnsureRootOrHalfCouncil;
 	type RegistrarOrigin = EnsureRootOrHalfCouncil;
+	type OffchainSignature = Signature;
+	type SigningPublicKey = <Signature as traits::Verify>::Signer;
+	type UsernameAuthorityOrigin = EnsureRoot<Self::AccountId>;
+	type PendingUsernameExpiration = ConstU32<{ 7 * DAYS }>;
+	type MaxSuffixLength = ConstU32<7>;
+	type MaxUsernameLength = ConstU32<32>;
 	type WeightInfo = pallet_identity::weights::SubstrateWeight<Runtime>;
 }
 
@@ -2208,6 +2214,9 @@ pub type Executive = frame_executive::Executive<
 	Migrations,
 >;
 
+// We don't have a limit in the Relay Chain.
+const IDENTITY_MIGRATION_KEY_LIMIT: u64 = u64::MAX;
+
 // All migrations executed on runtime upgrade as a nested tuple of types implementing
 // `OnRuntimeUpgrade`. Note: These are examples and do not need to be run directly
 // after the genesis block.
@@ -2215,6 +2224,7 @@ type Migrations = (
 	pallet_nomination_pools::migration::versioned::V6ToV7<Runtime>,
 	pallet_alliance::migration::Migration<Runtime>,
 	pallet_contracts::Migration<Runtime>,
+	pallet_identity::migration::versioned::V0ToV1<Runtime, IDENTITY_MIGRATION_KEY_LIMIT>,
 );
 
 type EventRecord = frame_system::EventRecord<