fix(rate_model): increase MAX jump rate and full rate

Recently all USDT on Polkadot was borrowed, leading to Moonwell USDT
borrow rates of 192% and Interlay $492k of USDT max borrowed, resulting
in Interlay USDT borrow rate pegged at 45-50%. The negative thing was
that there was no remaining USDT lending liquidity for lenders to
withdraw their capital.

Interlay referenda 96 was approved to adjust the USDT/USDC lending
params to set the jump rate to 20% and the full rate to 150% with
optimal utilization of 80%. After community and team discussion, this
solution was agreed to use the higher full rate to push lending back
down towards the optimal lending utilization.

https://interlay.subsquare.io/democracy/referenda/96

Unfortunately the referenda failed to execute because the 150% proposed
full rate exceeded the hard-coded max value of 50% for the full rate.

https://interlay.subscan.io/block/4905046?tab=event&event=4905046-0

In this commit I recommend:
* leave 10% as max base rate
* increase "jump rate" hardcoded max from 30% to 100%. We were already
considering 20-30% for USDT.
* increase "full rate" from 50% to 500%.

I definitely support having limits since people might submit a value
with wrong number of zeros accidentally. But let's place the limits
higher than any "reasonable" value that we might set in governance.
I can see us potentially having a market with jump rate 50% and full
rate 300%.

crates/loans/src/rate_model.rs

@@ -113,8 +113,8 @@ pub struct JumpModel {
 
 impl JumpModel {
     pub const MAX_BASE_RATE: Rate = Rate::from_inner(100_000_000_000_000_000); // 10%
-    pub const MAX_JUMP_RATE: Rate = Rate::from_inner(300_000_000_000_000_000); // 30%
-    pub const MAX_FULL_RATE: Rate = Rate::from_inner(500_000_000_000_000_000); // 50%
+    pub const MAX_JUMP_RATE: Rate = Rate::from_inner(1_000_000_000_000_000_000); // 100%
+    pub const MAX_FULL_RATE: Rate = Rate::from_inner(5_000_000_000_000_000_000); // 500%
 
     /// Create a new rate model
     pub fn new_model(base_rate: Rate, jump_rate: Rate, full_rate: Rate, jump_utilization: Ratio) -> JumpModel {