feat(slack): add reminder on postmortem incident

[jennafauconnier]

Implement Postmortem Reminder Notifications in Slack

Description

This pull request introduces a feature that sends reminder messages for incidents requiring postmortem updates in Slack. It ensures that reminders are sent to the commanders and the appropriate channels based on the incident's status and the time since the last update.

Changes Made

• - Added functionality to check for open incidents that are not resolved (i.e., those with _status values between FIXED and CLOSED).
• - Implemented logic to check if reminders have been sent:
  - If a reminder to the commander was sent more than two days ago and no channel message exists, a new reminder will be sent to the incident's channel.
  - If no reminder has ever been sent to the commander and the last incident update was over two days ago, a direct reminder will be sent to the commander.
• - Optimized database queries for fetching incidents and their related roles.

Motivation and Context

• These changes are important as they improve communication and efficiency in incident management. By sending timely reminders, we ensure that incidents are addressed appropriately and that commanders are kept informed of their responsibilities. Being more efficient on post mortem management.

How to Test

1. Trigger the send_postmortem_late_reminder task in your environment (use ff-manage task <name_of_task>).
2. Ensure that there are open incidents that meet the criteria for sending reminders (i.e., not resolved, priority is 5 or less).
3. Verify that reminders are sent based on the following rules:
   A reminder is sent to the incident's channel if the last reminder to the commander was sent more than two days ago.
   A direct reminder is sent to the commander if no reminder has ever been sent and the last incident update was over two days ago.
4. Check the logs for any errors or exceptions to ensure that the functionality works as intended.

Checklist

• I have tested my changes
• My code follows the style guidelines of this project

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 29.85075% with 47 lines in your changes missing coverage. Please review.

Project coverage is 62.55%. Comparing base (60a7836) to head (68acc72).

Files with missing lines	Patch %	Lines
src/firefighter/slack/tasks/send_reminders.py	24.13%	22 Missing ⚠️
src/firefighter/slack/messages/slack_messages.py	45.45%	12 Missing ⚠️
src/firefighter/slack/tasks/reminder_postmortem.py	11.11%	8 Missing ⚠️
src/firefighter/slack/models/user.py	28.57%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
- Coverage   62.83%   62.55%   -0.29%     
==========================================
  Files         215      216       +1     
  Lines        9587     9677      +90     
  Branches     1289     1305      +16     
==========================================
+ Hits         6024     6053      +29     
- Misses       3354     3415      +61     
  Partials      209      209              

Flag	Coverage Δ
unittests	62.55% <29.85%> (-0.29%)	⬇️

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[NicolasLafitteMM]

/rerun