feat(slack): invite specific Slack user groups for P1 incidents

[jennafauconnier]

feat(slack): invite specific Slack user groups for P1 incidents 

Description

Need to add a case when an incident is P1, we have to invite automatically a specific groups of user

Changes Made

• Add tag field for UserGroup model
• Make a migration to apply this field
• Add a new receiver to add specific group when an incident is P1
• Reorganize the back office presentation of a user groups to be more ux/ui friendly
• Add a comment into Group model to be more clear. (linked comment for docs)

How to Test

Create a user group with you and others people, create an P1 incident, and check if all users of this groups are invited to this incident.

Checklist

• I have tested my changes
• I have updated the documentation (if needed)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 52.63158% with 9 lines in your changes missing coverage. Please review.

Project coverage is 62.85%. Comparing base (86e516c) to head (6c7c979).

Files with missing lines	Patch %	Lines
src/firefighter/slack/signals/get_users.py	33.33%	8 Missing ⚠️
src/firefighter/incidents/models/incident.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #110      +/-   ##
==========================================
+ Coverage   62.82%   62.85%   +0.02%     
==========================================
  Files         215      215              
  Lines        9572     9587      +15     
  Branches     1286     1289       +3     
==========================================
+ Hits         6014     6026      +12     
- Misses       3349     3352       +3     
  Partials      209      209              

Flag	Coverage Δ
unittests	62.85% <52.63%> (+0.02%)	⬆️

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