Added a settings variable to allow configurable time periods for notification deletion

[arilloid]

Proposed Changes

• Added NOTIFICATION_RETENTION_DAYS variable that defaults to 30 in the config/settings/base.py under "Push Notifications":

• Modified the delete_old_notifications() to use the settings variable instead of the hardcoded time period

• Modified the test_delete_old_notifications() to use the settings variable instead of the hardcoded time period

Associated Issue

• Fixes: #2528

Merge Checklist

• Tests added/fixed
• Linting Complete

Only PR's with test cases included and passing lint and test pipelines will be reviewed

@ohcnetwork/care-backend-maintainers @ohcnetwork/care-backend-admins

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.00%. Comparing base (00cd5f9) to head (44c6912).
Report is 12 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2547   +/-   ##
========================================
  Coverage    70.00%   70.00%           
========================================
  Files          198      198           
  Lines        11660    11662    +2     
  Branches      1604     1604           
========================================
+ Hits          8162     8164    +2     
  Misses        3107     3107           
  Partials       391      391           

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