Implements a variable deposit base calculation for EPM signed submissions (#1547)

**Note**: This is a lift-and-shift PR from the old substrate and
polkadot repos, both PRs have been reviewed and audited
(https://github.com/paritytech/substrate/pull/13983,
https://github.com/paritytech/polkadot/pull/7140)

---

This PR implements a generic `BaseDeposit` calculation for signed
submissions, based on the size of the submission queue.

It adds a new associated type to EPM's config, `type SignedDepositBase`,
that implements `Convert<usize, BalanceOf<T>>`, which is used to
calculate the base deposit for signed submissions based on the size of
the signed submissions queue.

`struct GeometricDepositBase<Balance, Fixed, Inc>` implements the
convert trait so that the deposit value increases as a geometric
progression. The deposit base is calculated by `deposit_base =
fixed_deposit_base * (1 + increase_factor)^n`, where `n` is the term of
the progression (i.e. the number of signed submissions in the queue).
`Fixed` and `Inc` generic params are getters for `Balance` and
`IncreaseFactor` to compute the geometric progression. If
`IncreaseFactor = 0`, then the signed deposit is constant and equal to
`Fixed` regardless of the size of the queue.

### Runtime configs

In Kusama, the progression with 10% increase without changing the
current signed fixed deposit is: (term == size of the queue)

Term 1: `1,333,333,332,000`
Term 2: `1,333,333,332,000 * 1.10 = 1,466,666,665,200`
Term 3: `1,333,333,332,000 * 1.10^2 = 1,613,333,331,200`
Term 4: `1,333,333,332,000 * 1.10^3 = 1,774,666,664,320`
Term 5: `1,333,333,332,000 * 1.10^4 = 1,952,133,330,752`
Term 6: `1,333,333,332,000 * 1.10^5 = 2,147,346,663,827.20`
Term 7: `1,333,333,332,000 * 1.10^6 = 2,362,081,330,210.92`
Term 8: `1,333,333,332,000 * 1.10^7 = 2,598,289,463,231.01`
Term 9: `1,333,333,332,000 * 1.10^8 = 2,858,118,409,554.11`
Term 10: `1,333,333,332,000 * 1.10^9 = 3,143,930,250,509.52`

Westend:

Term 1: `2,000,000,000,000`
Term 2: `2,000,000,000,000 * 1.10 = 2,200,000,000,000`
Term 3: `2,000,000,000,000 * 1.10^2 = 2,420,000,000,000`
Term 4: `2,000,000,000,000 * 1.10^3 = 2,662,000,000,000`
Term 5: `2,000,000,000,000 * 1.10^4 = 2,928,200,000,000`
Term 6: `2,000,000,000,000 * 1.10^5 = 3,221,020,000,000`
Term 7: `2,000,000,000,000 * 1.10^6 = 3,543,122,000,000`
Term 8: `2,000,000,000,000 * 1.10^7 = 3,897,434,200,000`
Term 9: `2,000,000,000,000 * 1.10^8 = 4,287,177,620,000`
Term 10: `2,000,000,000,000 * 1.10^9 = 4,715,895,382,000`

and in Polkadot, the deposit increase is disabled in the current state
of the PR, as the increase factor is 0% -- so nothing changes from the
current behaviour.

Closes https://github.com/paritytech-secops/srlabs_findings/issues/189

substrate/frame/election-provider-multi-phase/test-staking-e2e/src/mock.rs

@@ -32,7 +32,7 @@ use sp_runtime::{
 	},
 	testing,
 	traits::Zero,
-	transaction_validity, BuildStorage, PerU16, Perbill,
+	transaction_validity, BuildStorage, PerU16, Perbill, Percent,
 };
 use sp_staking::{
 	offence::{DisableStrategy, OffenceDetails, OnOffenceHandler},
@@ -47,7 +47,8 @@ use frame_election_provider_support::{
 	SequentialPhragmen, Weight,
 };
 use pallet_election_provider_multi_phase::{
-	unsigned::MinerConfig, Call, ElectionCompute, QueuedSolution, SolutionAccuracyOf,
+	unsigned::MinerConfig, Call, ElectionCompute, GeometricDepositBase, QueuedSolution,
+	SolutionAccuracyOf,
 };
 use pallet_staking::StakerStatus;
 use parking_lot::RwLock;
@@ -182,6 +183,9 @@ parameter_types! {
 	pub static TransactionPriority: transaction_validity::TransactionPriority = 1;
 	#[derive(Debug)]
 	pub static MaxWinners: u32 = 100;
+	pub static MaxVotesPerVoter: u32 = 16;
+	pub static SignedFixedDeposit: Balance = 1;
+	pub static SignedDepositIncreaseFactor: Percent = Percent::from_percent(10);
 	pub static ElectionBounds: frame_election_provider_support::bounds::ElectionBounds = ElectionBoundsBuilder::default()
 		.voters_count(1_000.into()).targets_count(1_000.into()).build();
 }
@@ -199,7 +203,8 @@ impl pallet_election_provider_multi_phase::Config for Runtime {
 	type MinerConfig = Self;
 	type SignedMaxSubmissions = ConstU32<10>;
 	type SignedRewardBase = ();
-	type SignedDepositBase = ();
+	type SignedDepositBase =
+		GeometricDepositBase<Balance, SignedFixedDeposit, SignedDepositIncreaseFactor>;
 	type SignedDepositByte = ();
 	type SignedMaxRefunds = ConstU32<3>;
 	type SignedDepositWeight = ();