[LOW] `flagResponse` can be issued for a prior invocation of the Orb

[odyslam]

Description

A holder can flag the response to the question of a previous holder.

• Alice is a holder of the Orb and triggers it. She receives an answer A, and she is happy about it. That happens on day 0.
• Bob purchases the Orb from Alice on the following day, day 1. He can call flagResponse, and give the triggerId of Alice's invocation.

This is because all the following checks will pass:

• A response exists for the triggerId
• The flagging period has not expired (7 days)
• The response has not been flagged

• 
  if (!_responseExists(triggerId)) {
  revert ResponseNotFound(triggerId);
  }

if (block.timestamp - responses[triggerId].timestamp > RESPONSE_FLAGGING_PERIOD) { revert FlaggingPeriodExpired( triggerId, block.timestamp - responses[triggerId].timestamp, RESPONSE_FLAGGING_PERIOD ); }

if (responseFlagged[triggerId] == true) { revert ResponseAlreadyFlagged(triggerId); }

## Suggestion

- Introduce a storage variable `flaggingEnd`
- In `trigger()`, add the following: `flaggingEnd = block.timestamp + RESPONSE_FLAGGING_PERIOD;` 
- In `flagResponse` check that: `block.timestamp < flaggingEnd`
- In `purchase()` and `foreclose()` add: `flaggingEnd = block.timestamp`

[lekevicius]

Wouldn't it be more efficient to just track lastPurchaseTime, updating that only during purchase(), and during flagging check that trigger time is after lastPurchaseTime? Fewer storage writes and probably easier to understand.

[odyslam]

Great suggestion. Will should do that.

[odyslam]

On top of this, we need to check that the response has happened after last auction, as an orb can change hands not only through a purchase but also through foreclose -> auction