Prevent repeat trip button working when slayer task is finished

[DayV-git]

🚀 Feature Proposal

When a monster is killed on slayer task, add a hidden is_slayer_task flag to the underlying trip, which can be tested when the trip is repeated. If the slayer task has ended, don't send the trip / send 'are you sure' message - and for bonus points, maybe roll a new task? though i guess that conflicts with patreon button. Alternatively, just don't have the repeat trip button at all in this scenario.