feat: send notification for joining/leaving an engagement (resolves #2578)

[jobara]

Resolves #2578

• Sends a notification to an individual when they join an engagement
• Sends a notification to an individual when they leave an engagement
• Added tests

Prerequisites

If this PR changes PHP code or dependencies:

• I've run composer format and fixed any code formatting issues.
• I've run composer analyze and addressed any static analysis issues.
• I've run php artisan test and ensured that all tests pass.
• I've run composer localize to update localization source files and committed any changes.

If this PR changes CSS or JavaScript code or dependencies:

• I've run npm run lint and fixed any linting issues.
• I've run npm run build and ensured that CSS and JavaScript assets can be compiled.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.27%. Comparing base (fd4bf38) to head (1880fdd).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@             Coverage Diff              @@
##                dev    #2614      +/-   ##
============================================
+ Coverage     97.25%   97.27%   +0.02%     
- Complexity     2290     2302      +12     
============================================
  Files           345      349       +4     
  Lines         10403    10511     +108     
============================================
+ Hits          10117    10225     +108     
  Misses          286      286              

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