Sherlock-85: Limit index isn't checked in repayDebt, so user control is void

[grandizzy]

Description of change

High level

• see https://github.com/sherlock-audit/2023-04-ajna-judging/issues/85
  • added _revertIfPriceDroppedBelowLimit for both repay and pull logic in BorrowerActions.repayDebt function
  • did check only once for both actions (different than suggested in report) in order to save gas when repay and pull at the same time
  • added unit test

Contract size

Pre Change

BorrowerActions          -  12,390B  (50.41%)

Post Change

BorrowerActions          -  12,390B  (50.41%)

Gas usage

Pre Change

| repayDebt                            | 576899          | 609055 | 598947 | 757317  | 16002   |

Post Change

| repayDebt                            | 587020          | 619176 | 609068 | 767438  | 16002   |