Add MaxTipAmount for pallet-tips (#1709)

Last week we experienced a governance attack.
Surprisingly, there was no upper limit on the tip amount.

Due to the mechanism of pallet-fragment-election, the council members
will be refreshed immediately. Attacker is easy to control the council
and give a large tip amount.

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

@@ -1289,6 +1289,7 @@ impl pallet_tips::Config for Runtime {
 	type TipCountdown = TipCountdown;
 	type TipFindersFee = TipFindersFee;
 	type TipReportDepositBase = TipReportDepositBase;
+	type MaxTipAmount = ConstU128<{ 500 * DOLLARS }>;
 	type WeightInfo = pallet_tips::weights::SubstrateWeight<Runtime>;
 }